Well when i was restoring the case of my first pc i decided to go on the easy way.
I managed to get one can of spray paint that matched almost perfectly the original color, and i covered some parts like chrome parts and buttons and LED with tape then i spray painted the whole case including the internal parts because there was some rust going on and as soon i removed the rust i painted to prevent.
But its not the better way to go.

You could try using super-fine (2000+) grit automotive sandpaper, preferably designed for finish coats and wet-sanding.

A few closeup photos in good lighting, with a ruler or something for scale, would probably help in determining a suitable method 😀

My guess based on the given information is that it'll land on some kind of filler and a recreation of the original structure though.

I've done it with putty for deep gouges then paint to match as best you can unless you own a booth and a gun then you can have them match the plastic for a can of paint 🤣.