Unless you are really trying hard to keep costs at zero, consider buying the universal system 80 board. They aren't that expensive and work great. Even if you repair the old one, this will let you play the game in the mean time.
https://ksarcade.net/rottendog-mpu080-replacement-for-gottlieb-system-80.html
The board is a little thicker than the original so it makes a much tighter fit with the connectors. If you get one, know that you have to really jam the connectors on there.