That area is critical as all the other areas. If your rules say you must use the step, then get new wheels is the simplest way. If you can't afford new wheels, then depending how bad the cut or scrape is, use either a 1/4 inch sperical or cylinder polishing bob and smooth it out. If it's not bad then use a mushrroom bob or a foam aluminum auto wheel bob to polish the step. Getting the step symmetric will be tough without a lathe using the polishing bob. A hand drill is good enough for the foam or mushroom. I'd advise getting new wheels until you have experience with the polishing and smoothing of plastic.