Gippeto wrote: idler and mounting assembly from a Case garden tractor...used to mount the mower.
The belt comes off the horizontal pulley (on the mower deck) takes a half twist, runs around the idler and tensioner pulleys, takes another half twist, and lines up with the (vertical) clutch on the front of the engine.
Exactly what you want to do...in reverse. V-belts with kevlar last longer for power transmission.
The set up he's referring to is commonly known as a "mule drive". Very common on 60's and early 70's models riding mowers. John Deere, Case, Cub Cadet (IH models), Bolens, Wheel Horse just to name a few all used them.
How ever your best bet would be to find a junked cheepo Montgomery Wards or Hechenger, Craftsman any riding mower with a transaxle drive. You will know it by seeing the V-pulley on top (once removed from the chassis, 4 bolts and some linkage and the belt) and it will have a shifter sticking out of it.
It's your best bet with the little engine your using, plus most of them are 5 speeds with reverse. All you need in 1 unit even brakes.