That drive is common to a bunch of GM starters- - - -chances are a local parts store will have it on the shelf. Another way to recognize a 12V starter is the shape of the drive lugs on the armature shaft. They're made in a slight spiral, while the 6V ones are straight. That turns the drive slightly as it engages the flywheel teeth and minimizes the possibility of teeth jamming together and failing to engage.
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!