You need to measure the damaged area first. If it turns out that the area is more than a quarter, you'll need epoxy for repair, while polyester resin will do for minor damage. Here you have to remember that the resin hardens best when the temperature is above 18 degrees Celsius in moderate humidity.
Also, because polyester resin is porous, it is not a good choice for fiberglass areas that are often submerged in water. You may browse https://panelstore.co.nz/products/fibreglass-repair-kit to get fibreglass repair kit.
Then you have to choose your preferred glass reinforcement. If the damage is large or in a structurally important area of the location you are repairing, you will need additional glass for repair work.
You can use a fiberglass filler for minor repairs, but fiberglass for major repairs. Use acetone to clean the damaged area after removing the missing piece. Then use masking tape to glue the repair area.
Then, according to the directions on the packaging, mix the resin and hardener in the correct proportions to produce twice the volume of the total repair area.
For fiberglass filler, you will need to mix the resin and filler together until the consistency is similar to peanut butter. If you are using a fiberglass pillow, take the part that covers the entire damaged area, then apply resin to both sides of the material so that it is saturated with resin. If you are only using resin, apply it until you fill in the entire damaged area and barely spill the material.