Power measurement, that depends on where you measured. Should be seeing close to this:
MPU IJ17 Pins 4,5,6 = +5.03 VDC @8.1 mV AC ripple
U26/U27 ROM Pin 1 = +4.91 VDC

If you measured 4.86 at the PSU you are going to probably find even lower at these points. BUT, the common symptom of low power is random resets and lockups. The failure you are seeing, my guess, is corrosion related most likely. If not, it could be another IC that is on the address/data bus that the RAM is having trouble with.

Yes. Even if you get the game to boot, you are likely going to have crashing/reset problems running that MPU at this voltage.

Think of things a little different. Your goal should not be to "just get the game booting"... It should be to make the thing reliable and play for years, moving forward.

Honestly, I would suggest to resolve the power problem by buying a new one or repairing that one, then remove the batteries from the game and try to power cycle it quickly a few times and report here if there is any change. Those are the easy steps you can do on your own before you should assume the MPU has issues.

Secondary issue to tackle later, your battery holder is likely not working unless you replaced it. You can buy a new one, mount it remotely or, if you are skilled enough to replace an IC, you can order and install NVRAM and you will never need batteries again.

Please post a picture of this board.

Yes, its bad enough. That corrosion is likely to migrate, or creep, into many of those components, and possibly already has. It could cause many problems. There is a few good threads on the subject of correcting it properly on this site, in order to repair and ensure reliable operation in the future. In a nutshell, removing all potentially affected components, carefully neutralizing and removing all traces of corrosion, and reinstalling new components. Not an easy quick fix for this series of board, but possible.