What kind of a drain plug is it, the kind with the bronze fitting on the hull that the plug screws into? If so I'd try getting some penetratinng oil into the threads to loosen any corrosion. If you really can't get it to budge (damaged threads?) then I'd think about removing the whole drain plug assembly from the hull and getting a new one but be careful to really bed the mounting screws well in 3M-4200 or boatlife caulk to keep water out of the transom. And I always put some OMC/Bombardier triple guard grease on the threads of the plug, comes right out even after 6 months in salt water...
Mine has this one....
http://www.jamestowndistributors.com/us ... o?pid=8168