Assuming it does not take any torque in use, just repair what you have. The difficult bits for which you think you need CNC are already there.
Drill and thread the hex part for at a size one or two less than the main stud (e.g if it is M8, tap it M6 or M5) and make up a shouldered threaded stud to go into that thread. Secure with loctite.
With care and a little thought, you do not even need a lathe – could be done in a drill press.