Without a multimeter you won't be able to do much. I mean a simple testing circuit made of battery+resistor+LED might do in a pinch but these days you can get a cheap multimeter for a few $ in most shops that carry electric goods like switches, sockets, fuses etc. Start with the keyboard socket and trace the connection back to the chip that controls it. Battery spill will turn all solder into porous, non-conductive state and you can tell by the loss of the shine you have tons of affected parts.

The good news is though the U5S CPU is completly compatible with 486SX. In fact if you can choose between SX and U5S, pick UMC, it's way faster at the same clock in most apps and games.
If the mobo has 2 different jumper settings, for "old" and "new" SX, aka -S parts (Intel added system management to their later 486 chips, including SX line), use the older variant setup.

I know they are cheap but somehow I never get to buying one. I know, a poor excuse 😀
Next month I hope...
Indeed a lot of solder looks really bad and chunky and it's a large area.
I think someone with experience can save this one but that's not me. Will give it a go in testing everything though.

Good news about my cpu then. I have a board which I still need to identify that holds a DX2 so that should work if I find the jumpersettings.
I asked because the SAW2 has a specific setting for the UMC. I'm going to check if it's the same for an SX in the manual.

Have you checked the back side? As the front side doesn't look so bad. You should check the tiny traces but none look like broken on this photos.

There was some leakage there too but it cleaned up pretty well. I'll need to look at it again when I have more light tomorrow.
I really thought the soldering looks bad on the keyboard IC and the 2 chips above.

If it were my board, I would remove all those components, replace the SMD passives, and clean and reflow the ICs with fresh solder paste. However, I guess that if you don't have a multimeter, you probably also haven't gotten around to buying a hot air rework station. 😉 By the looks -- how nasty the solder looks, and how far it spread -- I would guess that even if you surface-clean it, there's significant risk of it continuing to degrade. It's not an impossible job if you have a decent soldering iron, but it certainly would have a better chance in the hands of someone with more tools and repair experience. Up to you whether you want to pursue it or find a different board to start with.

Hehe I have one of these Chinese hot air thingies but haven't tried it tbh. My soldering is also ummmm almost non existent 😀
The multimeter will be for next month

I know someone who can do it for me but the costs will be pretty high for no guarantee. This board is ideal for someone who had time,skill and patience. I'll try to find info about my spare board for now.

Small update.

I managed to set the LED display.
If anyone would come across it, you can configure it like an S-501/F502

The Soyo board is back in the stock but I can do a swap for this board. If anyone knows which one it is...It has a Contaq 82c596 A chip. I searched on Stason for this chipset and one board came close but not exactly the same.

VLB SLOTS 2020

Hmmm that doesn't give me a result 😒

*EDIT*

So I got the board and decided to try and identify it through the BIOS string and I am stuck again.

It's an AMI BIOS

Now if I look up the third string, 1608, I get CMC. Going from there,according to Wim's BIOS, there is no board with the Contaq Chipset:

https://www.wimsbios.com/biosupdates/cmc.jsp

If I enter the last string, I get this:

http://www.elhvb.com/webhq/models/486vlb2/clt486vl.txt.html

The same string except the third part. I stumbled on this board before on Stason but the layout is different.

Can anyone help me further and explain how this is possible?
I suppose the jumpersettings for the CLT486VL won't be the same so I'm not going to change them and guess some settings.

Small update 😀

I finally bought a cheap multimeter yuy! I did some fiddling since I am a complete stranger to this but what is the best way to tackle the keyboard error now?

Anyone can help me with the identification of the motherboard? It runs my UMC fine but I would rather see it on the John Doe one.