Its tricky, but not impossible to weld these cast aluminum cases and heads. Traditional TIG welding tends to bring up a lot of porosity in the cast parts, so extra care must be taken. If you need to "build up" an exhaust stud boss, then layers of filler with peening in between is needed. Some times, its easiest to fill the entire stud hole, then re-drill and tap the hole. All depends...
I don't know a shop local to you, but I do have a very good machine shop local to me in Richmond, VA. You could reach out to the folks at CycleX (who are in your neck of the world) and see if they are able to do the welding repair, because they certainly can do the machine work.