The ridges that are worn smooth can be refreshed. Sometimes the grooves are filled with what used to be the peaks. Scrape the junk out of the grooves. If the surface has been simply worn away and there really are grooves no more, then you can use a triangular file to scrape new grooves. Radial grooves are the only ones you should make. And not too deep. And the entire process isn't the 1st solution you should try. It's not a real good idea to weaken a backplate. Not a real risk, but if you over do the scoring it could become one.

That slippage can actually be a symptom of your 4stroke being run on the lean side. They have a tendency to backfire more when the needle setting is off somewhat. And they'll do it in the air and not always very obviously. Most people don't do as great a job setting 4stroke needles by ear as they think they can. And it's not too hard to be off enough to cause slightly rough running. It's just as hard to hear a 4stroke running slightly rough as it is to tell the rpm of one on the ground.