There's a link here somewhere where a guy did just that. He used two rollers. One for the drive laid against the side-wall and two free rollers on the other side. I believe that on his design the driveshaft was the actual roller and he had a sort of lever spring loaded acted like a clutch pivoted the engine to engage and disengage.
With some thinking it wouldn't be too hard to use a hand clutch and have the spring take the pressure when it's engaged and the hand clutch when pulled disengages it.
Yea lawnmower engines have some power,man.
Ha! Here it is: