[Lynn Eades]

You could buy a small alternator that could have its "sticky out bits" machined off to fit it into your housing, a source being the likes of small Fiat tractors, and some of the small Japanese power packs.----Just some more thoughts----
A 6 volt geny can be converted to 12 volt by installing 12 volt field coils. The 6 volt armature can handle it, but a 12 volt armature is not up to it in a 6 volt application.
Most "run of the mill" batteries sold these days are "calcium" batteries (last 5 years or more). They tend not to work well in a generator application, so when you buy a battery for your old pride and joy, fitted with a generator, it would make sense to buy a suitable battery.