leaning towards cracked solder on headers.

the Williams power supplies and transformers had a weakness on the 22.6V wiring where the 3 wires pull too much current and that part of the plug burns up. I modified all the transformers on the games at work by doubling these wires off the transformer and using the adjacent pins, as the 3 wires have 3 unused pins next to them.
incidentally, these are the AC feeds to the power supply that are used for the +5 DC circuit. I would think if the +5 was completely knocked out though, you wouldn't be seeing 1's, you would have a blank screen. in stock trim, there's no adjustment for the +5 on these, so if the components go out of tolerance and dip below a certain point, you're looking at a power supply rebuild.
Moon Patrol being an edge connector game though, you can follow the PSU guide in my signature for how to clean the edge connector. if dirty enough, you'll lose voltage between the power supply and game board to where the logic will malfunction.
alternatively, you can try reseating the CPU. I would reseat every chip regardless.