That's a terrible bike to motorize.
Heck I wouldn't trust that bike stock going downhill with just a coaster brake!
I just finished these two Motorized Fat Tire Shifters and it's all about brakes to stop these beasts.
I suggest you return that virtually useless Schwinn and spring for a 7-speed Sun Crusher if you want a gas Fat Tire.
The dual V's work unusually well and since the best way to motorize a Fat Tire is to tie into the pedal drive, you might as well get a bike with gears.
It's a big strong cavity I'll bet even a 49cc 4-stroke shifter would fit in.
The key again is a jackshaft (shift kit) to move your engines output to the other side and then replacing the pedal system with a Freewheel mounted pedal system.
Your pedals are like a socket wrench handle...
You can turn the socket with the wrench but you can also turn the socket with your fingers and the wrench just ratchets.
The engine output sprocket is your fingers in that example.
Not what you wanted to hear I'm sure but I know of what I speak, that Schwinn will only bring you misery if you try to put that engine in it.