You wouldn't gain much by tryng to tap a hole that is already too large because it is stripped. First try wrapping cotton cord around the threads of the vent to see if the cord can seal what threads that remain to hold the vent as it is screwed in. Some pliable setting muffler seal might be placed into the hole, let set, and tapped. Or else drilling to the next larger size that will accept a brass plug to be drilled and tapped could suffice. Beyond that drilling to 1/4-inch tap size and installing a bushing will do. To weld, drill and tap is risky as cast-iron has to be heated evently.