Please help - Black water holding tank damaged

I'd suggest to grind/sand the broken fitting area to as flat as possible. Treat the tank area that is to be patched with a good cleaner/primer. Install an ABS fitting with flange over the hole in the tank, using ABS cement. The ABS cement is a solvent welding adhesive. It actually melts the pieces it comes in contact with. Be scrupulously clean. The advice to use a rubber coupler has a lot of merit. It may save your new work from suffering the same fate as the original.