Dean - isn't it just the small drive gear at the end of the starter that needs to be specific to your car? As in, would an Alfetta starter work but with the correct drive fitted? And maybe the drive on the current starter can be fitted or modified to fit?
Just thinking out load.....
Alfetta starter has lightly different mounting holes, its only a mm different but its enough to strip your ring gear, which is a pity as its a more powerfull 1.1hp vs .7 hp bosch. Could be made to fit by redrilling holes though.
I have a Paris Rhone starter from a 75 which looks good you could use for parts, I also have some Bosche starters.