Its friction that holds them in.  You will need to get out the broken piece from your existing visor.  I drill a hole and then screw in a long sheetrock screw.  Then take pliars and pull on the screw head.  It may come out on one shot or you may have to drill and pull a couple times.  It just depends on how fragile the plastic is.

To remove a good pin, I take a rag and some pliars and pull.  You have to make sure the rag is thick or you will mar up the plastic.  Else if you do not care about the doner visor, I would just cut the metal shaft out of the visor and pry the tube apart to release the pin.

Then you need to go to one of the early Mustang parts suppliers and buy some new rubber ends.

Good luck