All you can do is cut back to thicker plastic and refill from there. Have you tried epoxie putty? Great for filling large holes, then use the white putty to fill any small imperfections.
Well , I can't because there is nothing under this plastic. As a basic putty I'm using Novol automotive finishing putty. Now I have Miliput , unfortunately it's too late.
Anyway , Skorpion is almost ready for some paint:
I found this seat in my stash. It fits nicely into cockpit: