I'm guessing your restrictor is likely plexiglass. I have had good luck with other plexiglass and plastics using a 2000 grit wet sandpaper. If the scratches are really deep, you may have to start with a 600 grit or so... Make sure you keep it very wet. I soak my sandpaper in water overnight as well. Sand lightly with circular motions over the scratched area. Once you are happy the scratches are removed, it will look somewhat cloudy. This is where a good buffing compound comes into play. I use a mild scratch remover such as Mothers or Auto Glym. Finally, a polish (I use autoglym for this as well) to finish the job. (This also is my answer for scratched plastic sunglasses)... As with almost everything someone tells me, I also test ideas in a small spot first...lol....Good Luck....