The wheel horse tractors that commonly had horizontal engines have friction clutch drives. In operation,one half of the pulley drives the transmission and rotates all the time with the engine. The driven part of the pulley is held in place with needle bearings that keep it centered with the drive bearing,and when the operator puts pressure on a throw lever, the second pulley moves against the first and the fiber clutch contacts and spins the driven shaft. My driven pulley also has twin vees cut into it,for different drive ratios that can be selected.
This clutch drive unit is a beast and will drive a 60" mower deck. They should last 1000 hours and the clutch wear surface is easily replaceable. They were used on up to 20 horsepower engines like my 1992 520 H. Possibly you could find a beater Wheelhorse with a blown engine and rescue one of these units?