I like the idea of using gasoline and electric motors, but not if you MUST have the gasoline to run the electric motors. Best to save the gasoline to charge the batteries when there is no sunlight. Carry flexible solar voltaic panels rolled up with a foldable frame work.
I have been thinking more about what it would really take for a bike that can be driven from city to off road mountains. There are at least a couple that use gasoline, but you can't keep gasoline for very long without it turning to varnish. [If you know of a way, let me know.]
There is still a lot of misinformation about lithium-ion batterys out there. Like “they do not last long” (very wrong, it depends on how the pack is built.) And a silent “field oriented” controller will give much better hill climbing power by automating the phase advance process so that the motor is driven at optimal timing; giving more thrust when needed. https://endless-sphere.com/forums/viewtopic.php?f=31&t=65031&hilit=phaserunner#p976242
To build a pack that will last longer you must think about what stresses batteries. Making them run cool is essential; that requires having a large enough C rate (more cells in parallel). The right way is to build a pack is to design in much more Amp hours than you actually need. Like I need twice the amp-hours my pack has just to get to town and back with out pedaling. And when hauling a total combined weight of 450lbs or more pedaling is useless. So I need at least 3 times my packs 700Wh capacity to keep my battery pack cool. 2100Wh and more like 2400kWh, yet even more would make them last even longer.
Samsung 29E cells are the longest lasting Lithium-ion 18650 cells but a little unpopular with speed bikers that can't handle any extra weight. The pack of 240 cells I am working on will be only about 8” deep by 9”x10”. Lithium Iron Phosphate [LiFePO4] will last even longer but you will need many more of them to get the Watt hours needed.
And if I were to drive up mountains and haul twice the weight, I may start wishing for gasoline. But good design will solve that problem; get a medium sized battery at first then after testing the bike fully loaded with a Cycle Analyst meter to know how many amp-hours-per-mile is needed, you can build a much larger pack. My largest Watt hours per mile use per trip was up to 75Wh/m recently, but usually never goes over 57Wh/m with my smallish 1500w at 52v nominal motor [3000w at 72 volts]. And you will need more like 6000watts for that much weight on steep terrain. Get a controller for each motor, with heat sensor so you don't burnout the Brushless motors. Two motors are better than one, especial if you must use hub motors set in wheels; because a wheel is like a higher gear ratio than you can build with a separate motor. Even if the hub motors can work at a slower speed.
You will need to calculate as many variables as you can; like what your lowest gear ratio should be, having two different low ratios would be a big help. Big 3 to 4” wide motorcycle tires and plenty of rack room to pack every thing all in one trip. You may need to get off the logging roads. So a short bike with a heavy duty trailer will be needed. And flexible roll-up solar panels with charge controller will be needed.
Knobby tires are not good for the road because they are nail magnets. I once got a large nail right through the middle of my MTB tire. But you will need traction off road. Forget about turning an old bicycle into one of these vehicles. You need to start from scratch and use motorcycle tubes and parts. Like stainless steel not aluminum rims and all airplane steel tubing. Don't build it like some kind of race machine, heavy duty tubing is needed. And extra low motorcycle gears. Better to crawl than to burnout your motor. If the tires will not stand up without air they are not thick enough. The first time I used my 4 ply front tire, it was flat down to 7.5psi And I did not even notice it. It was nice soft ride but I was able to go to the food bank and bring home about 40lbs of food [total combined weight of at least 400lbs] so a round profile steel belted 4ply would be the thinnest tire to use. And large spoon shaped tire irons are the best thing for removal of tires. Don't even try bicycle tire irons.
I need some one to build a frame that will hold a large battery pack in the center of the frame so it won't bounce around. But I still need a usable crank with chain drive and large box rack on the front and rear. And the rear should have a 16" rim 2.75" to 3"wide motorcycle tire with 2.25" on the front. The rear rim should be a steel motorcycle rim, while the front could use an aluminum BMX rim. Although I am not sure there is any point in using an expensive bicycle rim. High tech metals seem absurdly over priced unless you are a weight weeny. And there are too many people brainwashed by the bicycle industry; resulting in people thinking they should build cargo bikes with one inch tires to lower the rolling resistance. Even though they will wear-out 10 times faster than a 4 ply motorcycle tire. A tire made for large amounts of weight will ride higher on the tarmac than a tire made for light weight, even at a higher PSI.