If the cylinders aren't ported and you don't have much in them just find another used set....way less hastle imo. It is possible to fix them but if you're asking these questions no offense, but it's ove your head.
There are 2 case plugs on the opposite side of where you slide the shift fork shafts through, they are more than likely in there. Figured I'd tell you since mike0chek didn't know.
The melted plastic had to go somewhere. What mods does it have? I'd check the jetting and perform a leak down test before you throw another impeller in it and ride.