I worked in and around J22 and J24s and have seen both the repair and new build process. I think that crack developed from too much weight on the keel on the trailer.
If I were in your situation, I would raise the boat up, loosen the keel bolts and move the keel down a little. Clean up the area between the keel and sump, and then re-bed the keel back to the sump, re-torquing all of the keel bolts, and also make sure the keel is aligned well with a level boat deck. Once the keel is tightened, they used a heavy biaxial fiberglass, if memory serves, to glass over the seam and then it was sanded and faired into the keel. So you'll need to start this process by grinding out the old glass or at least grinding down to the sump/keel joint first.
After all of this, you'll want to take a little pressure off of the keel with increased pad height or more padding to lift the hull a touch.