Is there enough room at the bottom to cut out the broken plastic, and install a bulkhead NPT fitting in it's place? These fittings are used to add NPT fittings to rain barrels, aquariums....
this would require making a hole at the top of the tank to get the fitting inside.
As a temporary fix, maybe you could put a hose clamp around the threaded body that sticks out of the tank to help squeeze the threads around the drain valve?