This comment was posted after a discussion on switching to 12 volt batteries and jumping with 12 volt batteries.
I run 6 volts with out any trouble, just replaced a 9 year old one when it would not hold a charge for more than a week. But if you want 12 volts check Service Motors, a few year ago had some Crosley generators converted to 12 volts, I don't know if they still have any or not.
If you jump your 6 volt Crosley the safe way is to put it in neutral with the switch on and jump to the starter. I have heard of a few 6 volt batteries blowing up if they are jumped directly. This puts the solenoid between your 12 volt battery and the 6 volt battery.