There are a lot of 12V Delco-Remy generators on Ebay, and some of them even work. You'll probably need to install a longer mounting bracket, since most of the 12V units had the stepped-down field frame that was a couple of inches longer than the 6V units. Check Ebay for 12V negative ground aircraft generators and regulators- - - -plenty of them listed, and they will have much lower running time than auto or truck units. The battery and the rest of the electrical system won't care if the vehicle rolls, floats, or flies.
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!