I would consider using epoxy for the repair as well, rather than fiberglass resin from the local Home Depot, because the resin there is not marine quality (at least, it is not where I live, in Florida, though people do it all the time for these sorts of issues), and the epoxy resin sticks to old fiberglass better than new fiberglass sticks to old fiberglass. The epoxy costs a little more, sure, but it works a LOT better in not coming loose later. You can get epoxy resin online in Ebay, Amazon, or a host of other places, and it sticks to hair and skin or clothing exceptionally well, better than fiberglass resin in fact, so wear protective gear as if you were using polyester resin and you will be fine. Another nice thing about epoxy is that the liquid does not have a high VOC content like polyester does, so all the product you apply stays in the repair, it does not evaporate because there are no solvents to lose. This makes the repair stronger because the matrix sets up in the same fabric you would use anyway with the fiberglass resin, but it is far more waterproof than polyester ever will be. Additionally, there is no need for gelcoat. Bottom paint and you are on your way to sea. I would consider adding a backing plate of roving and epoxy inside the hull as well, both before and after the keel (once you determine where the likely damage aft is located). All in all, you are looking at about a half day of dremel work, a period of time of inspection and clearing interior to access the hull for that inspection (dependent upon what has to be moved), a full day of easing the external hull surface back a bit to allow the addition of more roving in the place of what you grind away in your layup prep, and probably a few very short days (of an hour or less apiece) to lay up the repair with the epoxy and glass roving fabric. Make the last layer something smoother and tighter to prevent severe displays of cloth texture at the surface so that fairing is far easier, and fair with epoxy as well for a very strong bond. It will be an excellent repair, and depending on your skills, will take a week or two to do, given the intermediate time needed between layers that are put on in 30 minutes or so per layer for the prior layer to gel enough to work over.
This is by no means an impossible fix. it is just that you have to have the intestinal fortitude to cut open the sole for the internal backing plate patch you will create (so the dremel would come in handy here again) and keep applying layers of material inside and out without shortcutting the process (thereby damaging the strength of the repair). Do not use CSM (chopped strand mat) cloth to patch this either, because it is a stressed part of the hull, and CSM is for making decorative turns (like a coaming) rather than any flat surface that is going to see significant stress in regular use, and it is also more prone to invasion by water leeched in through the glass of the original hull. Use single layers of e-glass that are oriented different ways at each layer, or better, bidirectional weave in each layer, layed up at alternative 45 degree angles. The cost will be the same as laying it up the same way, the labor will be about the same, but the results will be far more resilient. You will then have an extremely solid vessel with actually a REDUCTION in weight if you use epoxy, rather than an addition if you used polyester or vinyl-ester. It is definitely not impossible to save this vessel, but it will take a minor application of funds and mostly sweat equity. It may be as cheap to repair as it is to get the surveyor, maybe even less, depending on where you get the epoxy (hint, I get enough epoxy to BUILD a few dingies entirely out of six gallons of product (2/3 resin, 1/34 hardener, or a 2/1 mix) off Ebay for something like 350 bucks or less, SHIPPED cost to Florida from Texas, and you would need far less than that for this). Remember that as epoxy does not evaporate away like fiberglass resin mix does, you can look at the amount to fill the area, add in perhaps ten percent for waste, and STILL have extra after the fabric is applied and the remainder can be mixed with any number of things to make a paste for filling, to include, of all things, wood sawdust or even flour from the pantry to fair the final appearance and make it easy to sand.