There are a few different types of motherboard used. I replaced mine and it was pretty straightforward. though it is held in there pretty tightly. The plastic clip is what was retaining my battery.
There is also a battery on the prism card you should replace. That is a little tricky.
Here is a repair reference site for P2K:
http://www.videopenny.com/pinball/pinrepair/pin2000/index.htm