Steve,
If you are missing the fiber button,that is an easy fix.I would pickup a few in case your first try is not successful.
If you have the brake style pto and you think you are missing parts post a picture of what you have.
For now here is how I install one on a non brake style.
Most times the Pto arm will still have part of it still there.Remove the grill and engage the pto and get a small punch to knock out the remaining piece.When you install a new one,clean out the hole with a rolled up piece of 220 sandpaper.Then slightly taper the stub on the button to help start it in the hole.Now some say to pull back on the pto handle to drive the button in the arm,more than likely it will break, I prefer using the turnbuckle on the pto to set it.Just loosen the lock nut and start pulling the arm towards the pto. If there is still some space between the button and the arm,just tap the arm slightly while the pto is still engaged till it is flush.
Now disengage the pto and adjust the arm until you can fit a matchbook cover between the buttons.
