A 6V solenoid operated starter running on 12 volts will be a short-term situation. The solenoid will hammer itself to pieces due to the very violent engagement of the gear drive. It will also have a tendency to burn out the pull-in winding of the solenoid.

It is possible to mount a 12V solenoid on a 6V starter so the flywheel and the starter drive will be compatible. The starter doesn't really care what voltage it operates on- - - -the solenoid does.
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!