Without seeing them, I guess the boards you saw are those barebones hobby kits that sometimes show up for building a motherboard from scratch. Very rewarding if you have the skill and time, but will require you to source all the components (chips, resistors, capacitors etc) yourself, and expect considerable electronics, soldering and diagnostics skills. In the end it might cost you more than a proper secondhand motherboard.
Even if it’s not one of those, you should be highly suspicious of any motherboard you get for a fiver.
Either way you still end up with just a motherboard and will need all the other components.
Your BIOS seems pretty standard. As others have said, throttling a pentium 4 down to XT speeds is just not feasible through the BIOS. You could conceivably disable all sorts of caches and advanced features for optimizing speed, but you’d be lucky to cut its cycles in half, let alone get overall performance down to the speed you’re targetting. It’s like trying to tweak the engine of a supercar to make it perform like a moped – that engine simply doesn’t want to go that slow, and you’ll probably end up with all sorts of unintended side-effects in the process.
As for moslo:
The basic version of moslo is supposed to work on practically any version of DOS. Just download it and give it a shot!
If it’s not right for you, or you don’t like that it’s just an evaluation copy, do a search for “MS-DOS slowdown utility”. There seem to be a number of them floating about. Just pick one that sounds good and try it out! They should all come with documentation explaining how to use them.
These programs can’t break your computer – at worst they might make it act a bit weird while they are running.
