Honesty, if you don’t plan on going fast, I would just buy a good mountain bike at Walmart ($150), get a 4 stroke engine kit with extended crank pedals ($160 on eBay), buy some good accessories for your bike like lights, a comfy seat, better throttle, rear view mirror, speedometer, basket, quality fuel line ergonomic grips, and cup holder ($50 or so), buy and mount a 48T sprocket for better torque ($15 on eBay), and a 3 liter gas tank since it’s way better quality than the trash Chinese one that comes with it and holds twice as much gas ($25 on eBay). Also carry some spare tubes so you don’t get stuck if you have a flat in the desert. At least in my opinion, you don’t need to go mounting huge 79cc or 212cc 4 stroke engines on a bike you don’t even want to go above 20 mph on. Not to mention that you’ll have to custom fabricate a lot of parts, run into a lot more issues getting everything to work properly, and that in some places (at least in Kentucky) anything above 50cc is considered a motorcycle and requires registration...
just my advice!