The head would need to be removed, you will not be able to weld that stud hole successfully without first removing the core plug and making the stud hole damaged area much larger so you weld to clean aluminium, you cannot just plug weld the hole and re-thread the hole.
Aluminium shrinks after welding so if you do try to weld without removing the core plug it will probably leak (if not already) or fall out after, the core plug hole will need re-machining to an over size as it will no longer be round.
You may be able to drill and tap to a larger size and locktite a blind hole threaded plug with a flange after machining a recess in the gasket face, to accept the flange area then machining again for core plug clearance.
You could always gob it up with one of the liquid metal products, wind a stud in and sell it on ebay as a mint condition YB head