Still not big enough for adv riders lol. Nobody thinks about it but gas vehicles do have ranges. My car has about 400 miles of range, maybe 430 if all highway.
But yeah, if op wants a gas electric hybrid: a portable generator, a rectifier and a controller/motor combo that play nice at 72v.
Hypothetically a small battery in parallel would let a motor draw more power than the gen can provide for brief periods like starting from a stop or up a stupidly steep hill.
Biggest problem is that weight and size come at a huge premium on a bike. The smallest generators are still pretty big and heavy for tossing on a bike rack.
A 4 stroke friction drive is light, cheap, simple and effective except in the rain. It would be relatively easy to adapt an e bike pedal assist system to a gas bike, but the power for the electronics is my biggest hurdle. Ideally the engine would have an alternator but the nominal draw of the system would in the tens of watts, so a small battery would be viable.
Biggest problem is that weight and size come at a huge premium on a bike. The smallest generators are still pretty big and heavy for tossing on a bike rack.
You can also run it without a battery but then you're not going to be able to draw more than the generator can provide. If you have a 700 watt gen, that's all you can push. The battery lets you pull more, and how much more and for how long is determined by the watt hours of the battery pack.
Its hypothetically more fuel efficient at the cost of mechanical efficiency because you only use the fuel for the nominal power not the peak power: Around 60% efficiency from engine to the ground but it only has to put out enough to top off the battery pack so you can use a much smaller engine.
Hybrids aren't new but batteries that can handle rapid discharge and charging while also being lightweight are relatively new, the prius did come out in 2001 I believe. View: https://youtu.be/ftMxCehD08U