I agree with Mike on the basics, but
I would scrape the vinyl layer off the underlying ABS, inside and out, for at least 2.5" around the tear. Then if I had West Epoxy, (which I do), I would tape over the tear on the outside of the boat, and then mix up a batch of epoxy and pour it into the crack on the inside. It would be best to thicken the epoxy with chopped glass strands, or colloidal silica, or both, but with the tape on the outside, the epoxy will not run through. (The chopped glass strands do add strength, though.)
Now you have to cut concentric glass patches, the largest just fitting the areas you have scraped free of vinyl. The next size of patch should be a little smaller, and so on, down to one just large enough to cover the tear.
Let’s do the inside first. Sand and clean the de-vinyled area. Get your three concentric patches ready, laid out on newspaper. Mix a batch of epoxy. It won’t take much. Lay the LARGEST patch over the repair area. Then wet it out from above by dripping or spreading small amounts of epoxy onto the cloth, until it is clear. Don’t use much. Lay the next SMALLER patch on, and push it down. It will start to draw epoxy from the largest patch, so you won’t have to add much. When it is wet out and clear, put the SMALLEST patch on, just covering the tear with a bit extra. Wet it out using the least epoxy you can. The layers should be clear, but the weave of the epoxy should stand out. If you want to, put some plastic food wrap, polyethelene, or wax paper over the layers, and weight down a little with a pillow. Makes the result smoother. But once the epoxy has set for a day, you can sand the borders of the patches so they are smooth. Don’t sand into the cloth itself. That will weaken it.
Now you do the same thing on the outside of the boat, except that if you’re getting good, do a four layer patch. Fiberglass is an excellent outside strengthener, better than Kevlar. Kevlar works well inside the boat, but is harder to wet out, and it does not wet clear, so it’s harder to see what you are doing.
This may sound like a lot of work, but in hot weather, it can all be done in a day. Large powerboat equipment places have West epoxy, fiberglass, and related supplies. Epoxy ain’t cheap, but it will keep a long time for other repairs, though it will thicken and redden a bit.
E-mail me if you have questions. I also would like to know how an ABS boat in LA got so badly torn.