Have you tried running that O'Reilly starter on 6 volts? I'd be surprised if it works. What is the diameter of the armature shaft ahead of the drive gear, and does the drive gear rotate as it moves to the engaged position? If it's actually a 6V starter, it would take me approximately 2 hours to adapt the solenoid in the picture to work with it.

"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!