Drill out the perimeter, remove the remnant and install a new one using 3M rigid parts plastic repair. Or weld a stud on the remaining piece and back it out. Even when you remove a stud, the hole gets damaged because of the stud's thread cutting design. I have to relocate the studs on repro spoilers because they are often installed incorrectly. You can't use just fiberglass resin for a repair, it has no strength.