All welding procedures are fine and great but can be a frustrating operation with oily cast aluminium. But big question: Where do you get the thread at last, who will machine this ? This particular thread is not loaded much, just holds the cap - and that can be secured with a thin o-ring to stay safe. So my idea is to find a metal repair putty and glue the cap or better an exhaust nut with putty into the tidied up hole. You get the thread modelled from the nut into the cured putty in the same time. Of course you have to prepare the thread with some wax or release agent and a little heat may help for first time unscrewing. I think this could be a very economic way for a minimally loaded joint, welding can be done anyway later in case the result of putty use proved unsatisfying.
Vic
Link to putty: Putty
Vic
Link to putty: Putty