On my latest contraption, I used a Tecumseh snow thrower and a caster from a drum dolly. The caster was about 3 1/2 inches in diameter with a 5/8 bore. The engine shaft also had a 5/8 diamter, so I just slipped it on the shaft, and cranked the retainer bolt down hard. Then I started the engine, and ground the caster down to about 2 1/2 inches. It works fairly well, but does like to slip in rain and snow. I use a happy time clutch cable to pull the caster against the tire, and a bungy strap to release it. The engine is rated at 4400 rpm, so it is plenty fast. The caster could be taken down a little more. Wiped out on an ice patch yesterday at maybe 25 to 30 and it wasn't straining. The tire doesn't seem to wear appreciably, and the caster is like $3.00 and half an hour to replace. Unfortunately I blew out the rear tire when I went down, so I had to drag a broken bike about 4 miles home while dripping rain and snow. Ego was bruised more than the old carcass, althogh I wouldn't recommend it as an Olympic event. Hard to convince anybody I did it on purpose.
My next scheme is to use a jackshaft scrubber set up with a pneumatic dolly tire. Thinking it might slip less. Smallest I found so far is about 6 inches, so it will have to be geared down. We'll see.