So if I want to convert to a 12 volt system I need the 12V flywheel?
No, The 6 volt starter will work fine on 12 volts. Just remember not to crank it too long, as that will cuse the windings to burn up over time.
If you want to put a 12 volt starter on, then you will need a 12 bolt flywheel. The 6 volt flywheel will work for a bit, but it will get chewed up pretty fast by the 12 volt starter.
However, the 12 volt 6 bolt flywheel is much easier to find.