It's impossible to put a 12 volt armature in a 6 volt housing- - - -the shafts are different size. A 6V starter could be partially converted by using 12V field coils, but the 6V armature and drive gear would still need to be used. The 6V armature shaft and drive gear are 5/8" diameter- - - -12V items are 1/2" diameter. The gear tooth angles and thickness are also different between 6 and 12 volts, and the drive lugs don't match up. 6V drive lugs for the gear are straight, and 12V drive lugs are angled so the gear turns slightly as it engages.
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!