I agree with 12 volt conversion. A common Delco 10SI alternator has been made to fit an H, or a Hitachi alternator from a late 70, early 80's Datsun/Nissan is a smaller case and has been used a number of times. The 3 wire versions are not difficult to wire.
Spend the bucks for a 6 volt battery now, then the generator or regulator goes south and you are stuck. Either a replacement 6 volt regulator (if you can find one that works) or generator overhaul will exceed the price of an alternator.
With 12 volt negative ground alternator you will have greater reliability and be compatible with modern vehicles and less confusing if it ever comes to a jump start.
12 volt battery now should be in same price range as 6 volt.