For a drain pipe, the condition of the threads is not as critical as if it were a pressure system. As long as the new fitting will tighten into the elbow, just use a thick sealant which will fill the spaces where you damaged the threads and tightenthe pipe. You can solder the new brass fitting into the elbow, but that could create another problem when you want to take it apart: I meant to say in the previous message was could I solder the brass elbow to a copper fitting so it wont leak? Sorry….