First of all get that battery off of the MPU. It's a time bomb. The cheapest replacement would be to use a 4 place AA battery holder and mount it in the backbox. Install a diode in one of the battery compartments.

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Another option would be to replace U8 and U13 with NVRAM's.

The rectifier board is repairable but NVRAM.WEEBLY makes a nice kit that easy to assemble and don't cost much more than the parts alone.

https://nvram.weebly.com/new-pcbs.html

Your boot up problem may be with the connectors. I'd suggest you replace all the connectors and headers on all the boards.

Costly ???

Time consuming ???

That game is 40 years old. If you want it to last another 40 years it's worth it. Otherwise you'll be chasing bad connectors from now on.

GPE has all the headers and connectors:

https://www.greatplainselectronics.com/

These crimping tools help:

https://www.amazon.com/HT-225D-Cycle-Ratchet-Crimping-interchangeable/dp/B007JLN93S

That's the first thing I do when I get a game.

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