Depends on the tractor, you may end up with a horizontal shaft engine... I have my eye on an old Bolens with a newer Briggs Vanguard 18hp horizontal. The tractor's a hydro- the pto shaft from the engine goes via driveshaft straight into the hydro pump/tranny/axle- it's one big unit. If not for the fact that I plan on using it for towing stuff around the yard, it's be useless for "project" duty. That said, you can't beat them for pulling stuff- retarded amounts of torque, I have used this machine before and it's a total mule. Lots of power, built like a brick outhouse, and no fear of cooking belts or clutches.