To the OP: while I think your idea is interesting and it is something I've been toying around with myself, I don't think you can make a viable business around this if that is your thinking.

There just isn't a big enough market for it. If the target is a hobbyist engineer, then Frankensteining a homebrew x86 PC might be appealing for some sort of FPGA-based kit or motherboard based around the system-on-chip that Hightreason posted. An FPGA based system would allow people to try other architectures and not be limited to just a 386/486 system. I'm not sure that most hobbyists are going to be able to homebrew a motherboard capable of running a 386/486 class system, so then you're looking at a custom run of these things, and unless your're talking several hundred or thousands of parts it will be very expensive to make. I checked that link to the 486 motherboard but didn't see any prices, so that's not encouraging.

My guess is that to someone very interested in the underlying tech and wants to sort of design their own system is going to be more
interested in the FPGA route, or some other sort of kit.

If the goal is just to play games and not build/design a custom computer, then there are much cheaper ways to do it.

There was this project http://www.fleasystems.com/flea86.html

Check out his other projects http://www.fleasystems.com/projects.html

It wont be cheap, but you will have to go down the FPGA rout , there is already a 286 core but because of the size of the processor I don't think it will get any better, ideally maybe have the cpu separate from the fpga ,there are plenty of cheap 386 cpus still available, have the fpga act a NB+SB+VGA , and 1x isa 16bit slot for sound and expansion?

Only problem is Voltage, you will need 5v IO for the CPU and ISA data line, modern FPGAs dont have that.

Edit: Seems Cyrix CX87SLC is still available from china http://www.aliexpress.com/item/CX87SLC-33QP-Q … 51-cb762fd1f969

One option that _might_ work, using a far number of bridge chips, would be to build up around the 86Duino Zero. A combination of PCIe-to-PCI and PCI-to-ISA bridges, or a PCIe-to-ISA bridge, might allow compatibility with old hardware too.

http://www.ebay.de/itm/KLEINER-COMPUTER-233MH … 6-/311587800331

That looks interesting.

Has anyone actually tried to have this done? Looking at that ESA motherboard, I'd like to see someone take it, ditch the ISA slots, and instead add onto the board the Reproduction ADLIB card from here: http://hackaday.com/2016/10/03/a-reproduction … age-sound-card/ It's an OPL2 ADLIB Card. I don't know how possible it would be to change the OPL2 chip for an OPL3 chip and then put the new smaller board into a laptop chassis. An i486 13" laptop with a microsized Midi In/Out and HDMI Video out would be perfect. Dosbox can't run 486 games fast enough even on a kabylake CPU with noticeable slowdown in Privateer in certain conditions. I'd be very interested in getting this project going.

Vortex86 and PC-104 based computers maybe, there are some affordable ones and PC-104 is ISA signal compatible.