Rear friction drive would be the simplest and easiest. If you do it yourself with a weedwhacker/chainsaw/edger motor, it can be as cheap as $100 or less
If you use a new Staton with Subaru or Honda 50 engine, installation is less than two hours but cost is $499-$769, not including shipping.
Whatcha going do, climb hills or ride flat ground?
How fast ya wanna go, 20mph or 30mph?