Since we are talking about engine ideas, let me toss this out there.
Suzuki AX100 Lifan clone.
Make a mount plate for the front and bottom mount, and use your existing seat tube mount and adapt it for the rear. It's only an 8hp engine out of the box and you can get a cheap exhaust for the AX100 motorcycle for it and modify it as needed.
Make a hand shifter for it and have 4 gears to work with. Instead of riding around shifting all the time you can treat it as a range box. Set your sprockets up so that second and third are you main gears for riding, then you have first for the mightiest of hills, and 4th if you are going to hammer out some miles.