I have a 1974 TY250 that I am cleaning up and restoring to service. First sign of issues was when I removed the bolts for the exhaust flange. One was 10mm and the other was 11 mm. Now neither one will go in. Is there enough meat on the head to drill them out and heli-coil them? Can I go to 12 mm or do I need to go up to 13mm? Thanks !!