Ok, I think I've found something much more likely.. :)
I was searching around and found photos of the FZ50 which has a motor very much like the FA. A quick google search for FZs and I found theres a Danish site which has a few performance parts for the FZ50. 45mm big-bore, carbonfibre reed valves and 16mm carb.
I'll try a bit more research, but this looks promising.