Just about any 12 volt GM generator will swap, you just need to change pulleys. Most of the 12 volt units ran the narrow belt, but it's a quick switch to install the wide-belt pulley off the 6V generator. Of course, you'll need the corresponding 12V regulator to work with the generator. That's a straight-across swap also, using the same wires.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!