"Car replacement" hybrid bike

I'm looking for a 4 season car replacement that I don't have to register or pay insurance for.
I made an e-bike out of a cyclone 3000 electric motor and a few KW worth of batteries. I get about 45 miles @ 30mph average in my hilly area. But my commute is 40 miles per day and I would like to go further as a car does.
I'm looking to add a gas motor into the mix to extend the mileage, I figure the electric portion can get the bike going up hills and get the bike moving quickly and work in symmetry with the gas motor to increase efficiency on both on long rides, but there are some caveats I'm uncertain about.
For starters I don't want the kind of motor system that I will need to constantly maintain and tweak. I don't know much about gas motors and I don't want to be messing with it all the time.
This has lead me to a GEBE 38cc motor here https://www.bikeengines.com/shop/4-stroke-bicycle-engine-38cc-silver-eagle-kit/ easy install, reliable, claimed 200mpg.
Here's where it gets messy, my electric system can go at highway speeds if I so desire it to, but this motor system cannot handle those speeds I'm assuming. Of course I won't be going 60mph 99% of the time but I'd like to at least stroll about @ 40 mph when I can and go a minimum of 30mph average.
This is a quote from their site.

*** You can damage the clutch when coasting downhill, if your speed exceeds top engine speed, on level ground. Engine runs too lean when over-revved, coasting downhill and/or pedaling downhill at excessive speeds – i.e. running Faster than engine is capable of pushing you on level ground!!!

It goes against my grain to give throttle a pull, as if to give engine the gas, on that downhill leg – BUT – it engages the clutch, which I Do appreciate, it lugs the bike down, reduces the speed, I don’t have to stand on my brakes, risking over heating rim and tires, may cause a blow-out!

Cresting the hill, if speed slows down to +/- 10 mph, clutch falls out of engagement, you Will be freewheeling down hill. If you aren’t using the engine, the belt will make ‘slapping’ sound – belt continues to travel around gear on drive shaft and ring, chance is increased that you could damage belt.

If you want to fly down hill at excessive speed, it would be better to turn engine off and remove belt from drive ring first. WEAR YOUR HELMET!

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So it would appear that there is a conflict with my design requirements. Which leads me to you, hopefully you experiences folk can lend me your wisdom.