I've started to look into this a bit myself... with not a lot of success so far TBH
While there's a few examples of eBikes (trikes are actually a bit more common) that are... "sort of" hybrids, that is they've done what you've mentioned - strapped a small portable gasoline generator & a battery charger to their electric ride, I can't help but think this isn't quite ideal. While they've reported extended range, the problem is that you're lugging around this "powerful" 50cc engine spinning a generator capable of roughly 1000w output: Honeywell 1,000 Watt 49cc Gas Inverter Generator
limited by the charger's output - while you'll extend your range, you'll still run the batteries flat & will be sitting there w/the genny chugging away waiting for a sufficient charge to build up.
Which causes me to question the entire setup, all that added weight for a perhaps marginal increase in range, I suspect it may be a classic case of diminishing returns *shrug*
A more interesting take would ofc be if that generator had 1000w DC
output (& an electric starter) that was capable of powering the electric engine(s) while simultaneously charging the battery bank during lower demand. So say, 50% throttle or less and you're running on just the batteries, @ 20-30% drain the generator kicks in and starts charging, @ 50-100% throttle you're running off of both
the battery bank and the generator's output (which would ofc be for a "limited" time depending on capacity of the batteries), and/or just the generator's output at a reduced speed while the batteries charge. Stick on a few solar cells & be easy on the throttle and that may be jus' the ticket...
I'm a bit of an electric n00b and haven't the slightest idea if this is practical or possible in our scale, I'll freely admit I'm jus' kinda daydreaming
I hope some of our electric guys come in here to set me straight