The heat of the welding will expand the bolt, making it easier to turn after it cools. This was a favorite when I worked at a FORD factory. No welder? Clean out the socket with solvent on a "Q"tip and put in a dab of your choice of steel filled epoxy putty immediately forcing in a new proper size Allen wrench. after it hardens it should unscrew. cut off the Allen wrench and throw away the plug. . . 😉
