The PRAM battery is a standard 1/2 AA battery. I belive the IIC* don't need the battery, but it's highly suggested if you're using it often since a lot of settings are stored in PRAM.
Just make sure you take it back out before you put the machine into storage. I need to go pull some pram batteries from my machines.
Have you recapped the board? Usually the PSU is fine (besides also needing a recap) but the issue is with the large axial caps. If they're not good then the soft power from the power supply won't work. And usually the PSU recap is super simple.
Plus it already is going to need a recap, all those old electrolytic caps fail.