What voltage is your system running now? If the battery and charging system is still 6V, a 12V motor will run very slowly, if at all. A 6V motor will run on 12V with a dropping resistor, but mount the resistor somewhere inside a duct or other area exposed to air flow, as it will develop a considerable amount of heat.
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!