Hi Angel
A very warm welcome to the forum
If you are thinking of using a bike engine, I believe there is a lo-cost option for a reverse gear.
To put it simply, it consists of a gear wheel that fits on the diff flange & is operated by a starter motor
I expect one of the bike engined builders will provide some better details at some point.
Any questions, ask away, no question is daft if you don't know the answer