This is a known problem with J30 masts that get used in heavier air. The repair is detailed in the Articles of Interest section on the J30 website.
I do not have experience with this repair specifically but I do know how to remove rivets. You drill them out. Start with a bit that you know is smaller than the shaft of the rivet you are removing. Drill out the center of the rivet. Increase the size of the drill bit until you have removed the rivet. This way you'll be sure not to drill out more of the mast than needed.
You'll need to have the crack in the mast welded, of course.
