Take it to a jeweler and have them remove the old clasp and grind the back flat. Then have them silver solder two pins (like tie tacks) on the back, as far apart as possible on the left and right. You could also have them do 3 pins on the back in a triangle, to give the top support.

The newest version of the Xmarks kilt pin has 2 clasp back pins like these and they work great!