Running a 6 volt solenoid engaged starter on 12 volts might cause solenoid problems. I'd suggest finding a solenoid with a 12 volt coil and adapt it to the 6V starter motor. That takes a little tinkering with the solenoid mounting brackets. The other option would be to locate a 12V solenoid type starter from a 55-up 12V passenger car with a 235 and swapping to a 168 tooth flywheel.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
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