If your Puch has DC power then by definition, it must allready have diodes to convert what comes out of the flywheel alternator which can only produce AC.
I think you are confused by the fact that you have a hot wire and a ground wire. This does not make it DC, AC is wired exactly the same way.
If you have no other way to test, put a diode in series with your headlamp, either way around and fire up the ped. If the light goes out or does not change, you have DC, if it dims significantly you have AC.
If it is DC I'll email you a buck! : )