Re: How fast how much weight etc etc
I had a 23cc "weed" engine (Bike Bug). It moved me 200lbs., and a Next bike up to about 20mph or so with a friction drive. I would suggest something a bit larger for your application, like at least 35cc.
If you are talking about a "preditor" from harbor freight, then yes, dozens here have used it.
Probably a better bet than a "weed" engine.