Just to complete the record, the outside diameter of the 6Vand 12V flywheels are different so you can't swap ring gears to match the starter drive, you have to swap flywheels.
FWIW 6V starters do just fine on 12V. They spin a lot faster on 12V so the back-EMF from the armature limits the current, i.e. they don't draw twice the current. That said, it's probably not a good idea to run the starter on 12V with a low battery hoping it'll start, in which case they will draw way more current.
