A sneaky trick is to fit a 12V solenoid to the 6V car-type starter. I'm pretty sure that's a bolt-on swap, and it won't overload the ignition switch "start" position by carrying a very heavy current to the 6V "pull-in" winding of the solenoid. The 6V armature and field coils will live a long time on 12 volts.
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!