What brand is the alternator? It would seem that the only conversion you would need to do would be to install a rotor with a 12 volt field winding instead of the 6 volt one. The stator shouldn't require any alteration. Is it internally-regulated? If so, the electronic regulator will need to be changed, also.
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!