If I understand it correctly, that's not the remains of a bolt, but the remains of the threaded portion of the entire mount that twisted off from the nut.
If it were mine, I'd just cut the stub off flush, then drill and tap a hole and use a bolt. I believe that all that threaded part does is hold the shock on. The non-threaded portion carries all the vertical loads and the threaded part just keeps the top half of the shock from walking sideways. Not much side-load at all.
At least, that's how I'D do it...
Good luck with it, whichever way you go.
Kirk