If you use a normal bicycle engine mount kit. and ditch the pedals/cranks and put a old razor electric motor parts or kit on the freewheel side(myn is 3speed HUB!). You can "soft start" like dude said. pop clutch. then fly on gas(maybee even charging battery's when running gas, if not freewheeled but then you have drag). put it into neutral and kill gas motor.(this is where i put it into 3rd gear
...) then use electric motor as over drive.
When i did this, it felt amazing at that speed with only the noise of electric motor and mini chain. I say go for it!
Unless you can afford a electric hub and/or very good battery's. Go hybrid/assist. toss a light 2-stroke in the project. i call myn gas/electric assistance. not a hybrid until its recycle-tive power.