The original 6V starter will live about 3 days past forever on 12V. Just keep the engine well tuned and feed it the right fuel mixture so it starts quickly when cold. As long as you don't have to run the starter for over about 10 seconds at a time the overload won't damage it at all. The "Super torque" starters we used on high compression round track racing engines were built with field coils and an armature for a 6V starter.
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!