Here is what I did when I thought e-bikes were legal in my state ( was untrue)-
HF 79 spinning a Ford alternator, charging a set of batteries, with a scooter motor powering the bike. Would go just under 20 mph BUT the engine was flat out unless coasting. I found out an alternator is about 50 percent efficient ( did a lot of asking and looking on the net for that info but could not find it).
Gas milage was dismal. Finally bought a scrap moped w/ registeration and went to full gas power and made the bike into the moped- registered and everything/ plate on the back- no problems.
Here is the video I did-
I don't know if that is clickable so search - gas/electric e-bike on youtube.