If the kit is still in production, you may want to contact the manufacturer. Many companies will send replacement parts.
If you don't want to wait, you may want to try a vacuum formed canopy from Squadron. A spare is included in many of these, in case of errors cuitting the canopy from the sheet.
I don't know if any sanding & polishing, followed by a Furure bath, would fix it.
You could also try the "plunge-mold" process, using a heated sheet of clear Theraform (0.006").
Good luck, & keep the thinner capped & out of reach when you're not using it !