you could go with a push on and hold buttom but best to use it through a relay cause the amps drawn will need a heavy duty switch or it will warn up things that should not be warmed up. I'm not yet playing with the electric bike but getting interested in them I want to try a hub motor on the front and still have a gas engine. both would be fun 3 ways for power.
Pedals,engine,electric.

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norms-iron-works.tripod.com
motor assisted bicycle engine headlights that work off of your white power wire. check it out