I don't know of any salvage yards you could pick up a roller, I've bought 4 different chassis/rollers off ebay, the last was aluminum. I would think you could get a roller for 900 no problem, especially if you would use the wheels from your 500. I wouldn't use a 125 chassis, if you do you'll have to cut the downtubes and move them to accomodate the 500 motor, the 250 chassis will accept the 500 motor with only a motor mount mod, ok a slight head mod as well.