Pedbildr,
I have an el burro bike. Kinda similar to the one you purchased. The tires i have on mine are 20x10 in the rear and 19x8 in the front.
A 5hp or 6hp engine will work, but its like putting a 4 cylinder in a mustang....
i purchased a 9hp engine, works much much better than the 5hp i had in there.
With a frame that size, and a rear wheel that big, a 5 or 6hp engine will work hard to get it going... i would go for a 9+ hp engine... i picked up my engine on ebay for $230 including shipping.... once you have your engine you can play around with the sprocket sizes to get the speed / toruqe you want.....