Oh, I see from the photos, all the big chip are on sockets. That good. Then it will be fairly easy to try the 40 pin chips.
But after that, a methodical and tedious examination with an oscilloscope may be the best hope of finding the dead chip.
I am in the process of ordering one today. My next plan is to check continuity between board and pins in all socketed ICs. I could use my multimeter to test the Power is correct to all the chips as-well. I have ordered a few MK4116-3 RAMs so I can try the "piggyback" method to test the RAM chips. (These ones should be compatible i presume?) None of the 4116s on my board get overly warm so I assume there are no shorts.
Steep learning curve but hopefully I'll come out at the other end with a working PET and knowing a lot more about electronics !