The fiberglass is actually glass fibers embedded in polyester resin. But you probably knew that.
You can get sheets of fiberglass fabric and cans of resin at most marine supply houses. Sand down the existing roof to expose clean resin. Then mix up some resin with the appropriate amount of hardener, saturate a piece of fabric in this, and stick it to the roof. You may need to run a roller over it to get rid of air bubbles. Pour a layer of resin over this to seal up the fibers.
The resulting repair might not be pretty, but it should be adequate.
That's the gist of it, but I'm no expert. Search YouTube for 'fiberglass repair' and you'll probably get better results.