If you go the fibreglass route, use epoxy resin and a woven cloth and not polyester and chopstrand mat. The epoxy sticks to anything and you can almost be guaranteed of a very good bond. Polyester resins are hit and miss bond wise when going over materials other than polyester. If those guards were mine, I do as previously suggested and make a mold to give the approximate curve to the repair area and layup the glass repair from the inside of the guard. Let it cure, shape it, fill it with filler or more glass as required, make good.
If you add some black pigment to the resin it may make the colour match of the repair very close off the bat.