It depends what you need to fit. Can you post some pictures, Fiberglass is not that bad to work with. You can use bondo hair if you need fix larger areas and bondo glass for the finish. If you have to cat and put it back together use resin and mats.
Here is a link how somebody fixed a complete bumper which was to wide.http://forums.pelicanparts.com/paint-bodywork-discussion-forum/422042-fiberglass-front-bumper-fit.html