I've just stumbled upon this info on wikipedia
http://en.wikipedia.org/wiki/Ricoh_2A03 (http://en.wikipedia.org/wiki/Ricoh_2A03)
I also realised that I have two spare playchoice boards here missing their gpu's.. however they have socketed ntsc version cpu's on them.. I'm thinking a 50/60 mod is not too far out of reach.