Order some parts and start repinning connectors.
Remove that battery and neutralize the acid.
The 42V and 60V have their own special ground, are you measuring from that?
The displays aren't working. Is the game still booting? On my Count Down, sometimes it wouldn't boot (or won't boot properly) until you reseated the connectors on the right.
Consider just buying a replacement MPU or LISY1 if you continue to have trouble. The original system 1 MPUs aren't really worth bothering with that much in my opinion.