It is common for the threaded section of the shock mount to break off if the bolt is seized. This is very easily repaired using an M12 x 30mm set screw and this set of guidelines:

 

• Clean back the shock mount.  

• Run an M12 nut up the set screw.

• Weld the threaded end to the broken shock mount.

• Run the nut down the set screw and cut off the head.

• When the nut is removed it will clean the thread where it has been cut.

• Spray with zinc primer.