Can get the flap discs at most hardware or tool stores, I use the 4 1/2" but have seen guys use 7" also. I have a tire machine I made that spins 1500 rpm or so, using a 1/4hp electric motor, half an old axle, wheel hub, and some pillow block bushings, all mounted to a 3' tall bench I can move around or take with me to the track if needed. It has a 2" pulley on the motor and a 6" pulley on the axle. Have another one I made that the axle connects directly to the motor, with a rear cassette and bearing for support and wheel hub on the end of it, which I built for a friend. Plenty way to do it. I have seen guys put an old motor on their kart also, bolt the wheels to the rear axle, and turn up the idle on the engine so that it turns 1700-1800 rpm, and resurface their tires that way on the kart. You would want to use an old clutch also, with engagement set low around 1400-1500. Old max torque clutches will work for that when they get worn out. You'll want to make sure your wheels are balanced before resurfacing the tires, at least I do, makes the job alot easier without the wheel and tire wobbling like crazy from not being balanced.