Hi there,

Depending how comfortable you are with soldering eletronics and so forth, you could check out this thread:
https://pinside.com/pinball/forum/topic/vids-guide-to-bulletproofing-williams-system-6

Either way, check it out, so at least you know about what should be done.

The boards are quite serviceable, and yes, the original drivers create an incredible amount of heat, which you can eliminate once you move to a more efficient mosfet. Still, this heat is not a symptom, it is expected and just how this was originally designed with the tech they had back then.

I'd recommend posing the specific machine issues to the Flash pinball club here on pinside.

It sounds like the machine is generally pretty close tho. I would learn as much as you can before going in and shotgun repairing, as you could spend a thousand $ in parts easily.. lots of people just like to replace anything and everything, but really you can typically pinpoint most issues with the right tests.