Sounds like a wild goose chase and a hiding to nothing.. If the bike is 6volt, it needs a 6 volt battery. 12 volt battery plus a series resistor will drop the output voltage to 6 volts to run existing lights etc, but the bike charging system needs to be disconnected. Easier to change the bulbs to 12 volt, disconnect the dynamo and recharge the 12volt battery at home. All a bit strange, a new one on me.
Swarfy.