If you can access the crack inside and out: I would grind out the crack to fresh, raw fiberglass. Then feather the length of the crack til it is thin at the crack to full thickness out 4 - 5 inches. Then feather it the same way from inside. Clean it well. Apply strong tape to the outside or which ever side is currently on bottom. Lay in a heavy mat fiberglass patch 1” bigger than the cut, and epoxy it. Let it dry. Remove the tape. Do the same patch for the outside. Now continue building up inside and out with successive patches 1” bigger than the last. You can apply each patch inside without waiting for the prior one to dry. I would let each outer patch dry, sand it, clean it, and apply the next. This way, you can save yourself some filling and fairing. Once you have the patches built out to the edge of your feathering, let it harden. Fair the area. Fill it with filler or thickened epoxy. Let it dry. Fair the area again. Fill it again. Let it dry. Sand it smooth. It should be ready for paint. You have to pain it. The epoxy will degrade in sunlight. I have used biaxial 7oz mat for cracks like that. It works well.