I would consider the following procedure
- file off any broken stud still showing
- drill and tap a hole in the centre of the old stud to take a length of studding the same spec as the broken stud
- chamfer the edges of the hole just created
- screw in a lenth of the right studding
- MIG weld the base of the studding the the surrounding metal, filling up the chamfer you created

Potential problems
- insufficient metal to tap
- welding hardens the metal and may fatigue
- welding may undercut the stud and cause a stress point

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Cheers, Neil