Unless someone changed the starter, the engine should turn over fine with a 6v battery. If not, then you have a wiring issue from battery to starter and/or ground. Or, the starter is needing attention. Most people don't bother changing out the starter from 6v to 12v as the 6v starter will work and last just fine with 12v. Also, most will convert to alternator when switching to 12v unless an authentic look is high on the priority list. Also, look for voltage markings on the coil.
