The studs for the cam cover screw into the cam journal caps. So you can certainly R/R one without removing the head. Remove the cam cover. Double nut the stud or grab it with vise-grips (since it's stripped it's already ruined). You should probably remove the cam journal cap (after removing the stud) then install a new stud. I suggest removing the cam cap because the hole for the stud goes all the way through the cap - you need to make sure that 1) there is no debris in the bearing area from R/R the stud and B) to make sure your new stud does NOT protrude too far and foul the bearing.