Sal.
If you use a 12v starter with a 12v flywheel, or a 6v starter with a 6v flywheel, you'll have no problem with the ring gear or teeth on the starter drive (the teeth match).
Using a 6v starter/flywheel with a 12v electrical system will cause the starter to turn faster and might wear out the starter motor if you crank for long periods. Many people do this with no problems.
