I would keep original flywheel and starter. I had 6V starter on 12V for years and as long as you don't grind on it for extended periods of time it will give no trouble. Truck will actually start much easier as it spins over faster. Now when I went to Hydra-Matic with solenoid starter, I found a 12V solenoid as they told me that the bendix would hit flywheel teeth too hard using a 6V solenoid on 12V. Bottom line is the 6v starter motor will be fine on 12V. A solenoid is a little different story. Makes no difference on a foot stomper starter.