Roto zip that crack out of there and put in a backer patch, fiberglass on top of the patch. If the crack is under the cockpit be careful not to cut into cockpit, there will be room close to the keel but don't cut more than about a 1/4 inch deep. The backer will be made of a couple of layers of fiberglass and you can use cardboard underneath that to help pull it snug against inside of hull. See attached page from Sunfish Repair Manual
Had the same issue on mine, but with a few failed patches on top. I cut the whole area out and took a wood shim and glued it to the inside as a temporary splint while I put in new fiberglass. I covered the shim in sheetrock joint tape for added grip. After that, I just patched normally with fiberglass matting and sanded. Never did get around to painting it though.....