Not generally, but if only the water dams ( tophats) are rotted you may get luck and be able to use a plastic repair kit. Otherwise big job to do it right.
Here is an easier repair I done on my Falcon. It should work on your Mustang the same if it’s not too far gone. When I got things open up to see the damage it was nothing as bad as the pictures in the other links. I use Por-15 putty to patch the holes.