The dropout has (had?) an integral derailleur hanger so it can't be replaced without replacing the entire dropout and that will require the services of a frame builder. Given those parts are aluminum even most frame builders won't be able to help. A welder whose good with heliarc and knows how to weld aluminum could probably repair that break.
Another alternative is to find a "claw mount" for the rear derailleur. That's a bracket the derailleur bolts to and is clamped under the quick release nut. Crude but passible.