the 5 VDC for the MPU comes from the filtered 5V regulator on the solenoid driver board which gets it's input from J3 pin 8 on the rectifier board which should be about 11.9 VDC (test point 3)

there are a couple of recommended ground mods to do and joining test point 1 and test point 3 together on the solenoid driver board

you must have J2 on the rectifier board connected as this has the AC power in and ground etc.

start with J2 only connected and you should get the same readings as posted by Billc479

as far as the mpu flashes.....can you post some high res clear pics of the battery acid damage?
guessing that U8 and/or its socket will need some attention at least, the acid needs to be neutralised.

a lot of work ahead, try reading https://www.pinwiki.com/wiki/index.php?title=Bally/Stern#The_Single_Largest_Culprit_in_MPU_death