I never dreamed that I will be doing such a project, but I started working on it starting last December.
My initial goal is to develop replicas of 2 different PC mainboards straight out of their reference designs.
The first one is Intel 440BX chipset uniprocessor reference schematic still available on archive.org Intel developer site (BXUPDG10.PDF).
The second one is VIA Technologies Apollo Pro 133A based VT5228C reference design detailed in their likely leaked Apollo Pro 133A and chipset design guide with VT82C686A.
I started to collect quite a few components needed for these projects, and every week I order more of the parts I need to manufacture these mainboards myself eventually.
I already got Intel FW82443BX and FW82371EB on hand (2 of each) and I also got VIA VT82C686A as well.
Because someone was selling it on eBay, I also ordered this rather rare IBM number marked version of VIA VT82C694X called 38L3456.
Someone was selling it on eBay and I got 2 of them for $24 each.
Maybe I should not use this rare marking chip, and I should find someone who can sell me NOS (New Old Stock) VT82C694X.
I finished going through KiCad 8.0 Getting Started tutorial a week ago, and I am now ready to work on adding custom schematic symbols obviously needed for these projects.
Anyway, I am a fairly slow learner, so it might take a while to develop the boards.

In the meantime, I seem to be getting bogged down hunting down components needed for the second phase of the project for about the entire past week.
I have already obtained 4 pieces of VIA Apollo MVP3, , 20 pieces of Apollo Pro (VT82C691), 4 pieces of Apollo Pro 133T, 6 pieces of VT82C586B, 5 pieces of VT82C686A, and 4 pieces of VT82C686B.
I also have 3 pieces of Apollo VPX (VT82C585VPX only; I still need to procure 6 pieces of VT82C587VP.), 1 piece of Apollo MVP4 (VT8501), 2 pieces of PLE133 (VT8601A), 2 pieces of PLE133T (VT8601T), and 2 pieces of CLE266 (VT8623).
Not really relevant to the topic, but VIA still seems to be milking VT82C586B and VT82C686B even today that both parts I got had date codes sometime in 2023!
I do not know who still uses VT82C586B today, but it appears to allow TSMC (VIA's main foundary) to keep running their 500 nm (0.5 um) process.
It has been in continuous production since 1994 or 95, and back then TSMC got little to no respect from the industry.
I hope they weren't remarked, but the chips appear to look unused.
For ALi (Acer Laboratory, Inc.), I got 2 pieces of Aladdin V (Date code 9911, Rev. E) and 2 pieces of Aladdin Pro II (1999 date code, Rev. E).
Aladdin Pro II came from a US based seller.
I still need to obtain 4 pieces of M1543C that I currently do not have.
I do not have anything for SiS at this point, but I will like to design a mainboard with SiS5591 / 5592 and SiS5600 / 600.
The SiS chipset's NOS availability is poor and some of the eBay prices seem ridiculously high, so it is not a high priority at this point.
Detailed databook was available until SiS630.
That being said, I will be interested in working with a non-integrated graphics chipset based mainboard only at this point.

I purchased most of the VIA, ALi, Intel (440BX chipset), and several clock synthesizer devices from China based sellers a week or so prior to the Chinese New Year.
That being said, the incoming Chinese New Year, Donald Trump's tariff "threat" against the $800 de minimus importation, and the 10% tariff on Chinese and Hong Kong origin goods unfortunately sapped my interest in obtaining more components including ALi M1543C.
That being said, I ordered $1 worth of discrete transistors (10 pieces of AMS1117-3.3V) that included shipping (SpeedPAK) from China (How can this Chinese seller make money from such a small order?) a week ago, and there was no tariff man / women collecting the 10% (now 20%) or 10¢ tariff at my door.
I heard it on a broadcast on CBC (Canadian Broadcast Corporation) about Trump tariff tax that one week after the tariff went into effect via a presidential executive order, USPS (United States Postal Service) has paused trying to collect the tariff since it is impractical to do so.
USPS is supposedly not set up to collect tariff from small parcels, so I ended up not having to pay the tariff.
I also received 44 pieces of 2 Mbit NOR flash memory, and there was no tariff.
That being said, the parcel seem to have been routed through a third country, so the item took more than a month to arrive.

I still do like obtaining certain items locally, but the reality is that many of the PC related components were manufactured in Asia, so whether I like it or not, I cannot avoid importing components from China / Hong Kong for all practical purposes.
I did managed to purchase FW82443BX or FW82371EB from a computer component liquidator located in the same city NVIDIA is located, and they were only $1 to $2 more than what I got from sellers in China.
That not bad at all, but the prices and component availability (broad lineup of relevant goods) is very good with Chinese sellers, plus shipping portion is very cheap compared to the sellers in the US.
That being said, I will prefer US based sellers for analog / power / passives like capacitors due to the greater chance of getting stuck with counterfeit or undisclosed refurbished / used components.
I already had one incident with getting stuck with obviously used VT82C596A (FW82371MB pin compatible south bridge chip) that was supposed to be new from one vendor in China.
How do I know this?
BGA balls eere partially damaged.
They needed to get reballed.
That being said, almost all items I got from China appear to be new since the items look unused.
Still, one sometimes get the same item with date codes being years apart.

One thing that really takes a lot of time is figuring out which clock synthesizer can be used with which chipset and its availability (I hunt for them almost everyday on eBay.).
Some clock synthesizers are not really available on eBay, and procuring them is harder outside of eBay.
I probably will need to purchase them from semiconductor component liquidators, and this tends to cost more and less certain than eBay.
It turns out I do need a lot of seemingly irrelevant and obscure analog discrete and power ICs, and many different kind of passive components.
I got several clock synthesizers that can be used by Aladdin V and Pro II from ICS and Winbond.
Because I am strictly following the Intel 440BX chipset uniprocessor reference schematic, I had to obtain exorbitantly expensive Linear Technology and Maxim power and analog ICs.
For example, I paid $7 a piece for Linear Technology LT1585ACM-1.5, $3.50 or so for LT1575CS8, $4 for Maxim MAX1617.
In comparison, I paid $10 for FW82433BX (440BX North Bridge) and $9 for FW82371EB (PIIX4E).
I can see why Linear Technology had the semiconductor industry's highest gross margins (as much as 76% at one point).

Actually, obtaining the relevant connectors is also hard since the most such components are already long out of production, and I still do not have a one parallel port and two serial ports stacked connector on hand.
In fact, I do not even have a double Decker PS / 2 connectors part yet.
Someone is selling this on eBay for $13 or so for 2 pieces.
That's a near gouging price . . .
I cannot source these basic connector type items locally since several electronics component shops have gradually disappeared due to what I call, 5 story wooden frame condo building gentrification boom that is ongoing around the region NVIDIA is located (I am not blaming NVIDIA for this.).
I just obtained 8 pieces of AMP universal AGP connector recently for about $8 a piece including sales tax and shipping.
I will order Slot 1 connector tomorrow for $6 a piece.
Anyway, I make progress everyday little by little on figuring out the right combination of components (i.e., which clock synthesizer / processor power management IC can be used with which chipset / platform) to use, and figuring out the obscure active / passive discrete components that I never had to deal in the past.

As for the second phase of designing my own original retro mainboard, the first mainboard I am thinking of designing will likely be a Pentium Pro mainboard with VIA Apollo Pro chipset.
Yes, that largely forgotten Socket 8 platform.
I spent a couple of hours figuring this out today, and it appears that ICS ICS9148-37 (often marked as ICS9148BF-37) clock synthesizer can work with Apollo Pro chipset.
ICS9148-37 clock synthesizer is really meant to be used with Apollo MVP3 chipset, but Apollo Pro and MVP3 seem to share a similar clocking architecture, to which requires the clock synthesizer to supply a clock for its AGP clock domain, and the system's AGP slot.
Intel 440BX internally derives its AGP clock from the host clock (i.e., FSB clock), so it does not need a separate AGP clock from a clock synthesizer.
440BX forwards its own AGP clock to the AGP slot.
VIA appears to have noticed the system level advantage of deriving the AGP clock from the chipset's own PLL that they appear to have adopted this scheme starting with the controversial (from Intel's perspective) Apollo BX (VT82C692BX) chipset.
I do not have the full databook for Apollo Pro Plus (VT82C693), but Apollo Pro 133 (VT82C693A) definitely changed the AGP clocking scheme (confirmed with the full databook).
This allowed mainboard vendors to use the same clock synthesizer proven with 440BX on their VIA chipset based mainboards.
Another reason to use this obscure clock synthesizer is to support 60 MHz bus clock Pentium Pros like 150 MHz and 180 MHz parts.
It appears at this point that most 100 MHz capable clock synthesizers for Apollo Pro chipset does not support the 60 MHz clock frequency setting, so I need to resort to this scheme of using a clock synthesizer meant for Apollo MVP3 chipset.
As for components, I got 9 AMP Socket 8 and 22 AMP VRM8 Header (the Socket 8 / Slot 1 version of VRM7 Header 7 Necroware recently publicized on his YouTube channel).

Lastly, I am not really into a money making type of an activity, so all of my retro mainboard designs will be open source.
I will eventually throw it over to a GitHub like public repository.
Besides, the hardware is 25+ years old, so I doubt it can ever be a business.
That being said, if someone is willing to pay something like $250 USD, perhaps someone else (i.e., not me) can assemble the mainboard as a for profit endeavor.
Of course, it is not really that profitable (i.e., 20% gross margin), however.
I do not know how much it takes to assemble a PC mainboard in the one of the highest labor cost region in the US where I live (i.e., located in the same region NVIDIA HQ is located), but if it can be done for $50 per board, maybe the mainboard can be sold for $250.
Last time I checked, JLCPCB can seem to make 5 pieces of 9.6" x 9.6" (i.e., rather large micro ATX form factor) impedance controlled 4 layer PCB for about $140 when I tried their online estimator.
ATX size will push the cost higher, but below $200 for ultra low volume of 5 pieces should be feasible.
Of course, I understand that this is a prototyping quantity, not a mass production quantity.
As for this one of a kind VIA Apollo Pro Socket 8 mainboard's BOM cost, I think it should come in around $80 to $100, of course, excluding the PCB cost.
For example, I got Apollo Pro for $4 each and VT82C586B for $7.50.
I got Socket 8 for $3 each and VRM8 header for $1.25 or so.
ICS ICS9148-37 may cost me $7.50 each, however.

That's all I want to say for now (sorry for writing so much), and I will look forward to anyone's feedback.