Moped Army

1) check to see if it is truly in fact capable of producing 12V. Disconnect all 6V accessories, loading it with, say, a 12V auto headlight bulb (you have to load it with something). Start it up, put a volt meter across the output and check the voltage at idle, slowly increase the engine rpm and watch the voltage increase seeing in fact if 12V is a reality.
2) if 12V is attainable install a regulator and do your 12V bulb conversion
3) if 12V is not attainable then you are going to have to either live with 6V, or change your lighting coil (either buying one, or making one).