6 Volt to 12 Volt conversion

I have a friend that has a starter alternator business, trying to rebuild an original 6v generator to 12v, but can't find 12 volt armature (has bushing shaft) and 12v field coils to fit, anyone have info on doing this. My car has been converted to 12v with an ugly old gm alternator and would look better back to generator converted, 6v unit is new never used.