My concern is the rom sockets. The NeoDiag will start the game board with the prom disable disconnected (not needed with the roms not in the sockets though). So the major difference between Diag and Game is the roms being accessed. A close inspection for bent pins or corrosion is in order. Also can you pull the PROM out of the socket by hand or is it difficult to remove with a puller. I'd probably replace those old closed frame sockets anyway.

Good job figuring this out!