stingray xl alternate rear wheel

My son and I are on the heavy side, I'm 220 the son is 300 so even with the large tire on the Schwinn we are a little too heavy. My plan is to build a heavier rear wheel and add a gasoline engine. Haven't been working on it lately, too many honeydoo jobs but you can see the frame so far and this is the back wheel. when the little donut spare tire is on the wheel the height is about the same as the Schwinn wheel. The plan is the donut tire wont go flat or wear out and we can go off road without ruining a 30 dollar bicycle tire. The hub is a 1 and 1/2 piece of tubing with wheel barrow bearings.