I believe there's a different inside diameter ring gear between the 139 and 168 tooth gears. Just put 12V to the 6V starter and move on. If you want to have the resistor bypass terminal on the foot switch, just use the 12V switch. It's a bolt-on swap to the 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!