I pulled the PCB out of the machine and connected it with my test harness to a CRT TV. After sitting in a garage unused since the 90s it was surely never going to work?
I couldn't believe my eyes - it booted up and everything seems ok. Maybe I won't need that spare PCB after all. There's a bit of an audio hiss under the sounds but that's it. I'm going to leave it running for a bit to see if it explodes. ;)