rkbonds,
From Push trailer experience, I can say that even with a really tall gear, you can go up steep hills fine with a slight pedal-start, but your clutch will slip and heat up at low speeds, even though you will have PLENTY of power on the hills due to your engine displacement. (cc)
Harbor freight has a 5.5HP OHV Vertical-Shaft engine that would go great with that gearbox of yours. Or just use any vertical-shaft lawnmower engine you find.
Most Riding mowers only go like 5-7mph, so you will need to gear-up your ratio to get higher speeds. Either a clutch with a 3.7inch pulley driving a smaller pulley on the gearbox, or better yet, go to a chain, using a jackshaft to raise your gearing. The sky's the limit!!!
Your mower gearbox will make it MUCH easier on your clutch, and the clutch will lock more too. Imagine using 1st gear to get like 5mph to start, and gear up as you accelerate!!! Also, it would make it easier to go the legal 20mph in a lower gear, then use the high gears when you want to ZOOM!!
You are lucky, GOOD FIND!
I WANT ONE!
Crazy Riding,
ZnsaneRyder