Putting fiberglass on it could put it seriously out of balance. You
might have to stick some epoxy/glass on the other end to rebalance it.
The right glue would depend on the type of plastic. If it's a low
surface energy plastic, the adhesive that would work would cost
a significant fraction of the replacement part. Otherwise, the only
problem is ensuring the adhesive and patch can stand the heat in
the washer. Does the replacement part indicate what plastic it's
made of?
Doesn't say, but it sure looks and feels like PVC.