Man, that stinks.
I suspect you can just clean it really well. There are various instructions around Cloudy Nights depending on how compulsive & careful you want to be. Since your filter isn't exactly pristine at this point, you may not need to be quite so careful.
When I need to clean a filter (or any optics, actually), I use a high-grade ethanol (the higher the proof and the fewer the additives the better), cotton swabs, Kimwipes and a bulb blower. First I blow it off thoroughly to remove any dust or loose debris. Next, I dip the swab into the alcohol and lightly brush it across the surface of the filter. I then blow air on the filter to dry off some of the excess alcohol and then very delicately blot & wipe the remainder of the alcohol off with the Kimwipe. I then blow again with the bulb.
After I do both sides of the filter, I'll hold the filter up to a bright light and spot treat any remaining smudges or dust on each side.
At this point, I put it into the wheel as quickly as possible and keep the thing covered if it has to sit around while I do other filters.
As for the scratch, If the scratch is near the edge, you're probably fine. See what a new set of flats will do for you.
Best of luck.