Do you know what the keel is made of? Lead or iron? If it is lead, you might be able to get away with sistering in some keel bolts from another design if your keel is large enough. When I was replacing the keel nuts on my C-27, I was exploring the possibility of having to sister in new keel bolts. Catalina sells a kit for the purpose that are self-tapping once you've drilled a pilot hole. All you need is a good drill and a breaker bar.
If your keel is a lead fin with sufficient depth (3+ feet), look at adapting something from another design as an option. Just run the new bolts in next to the ones that broke off.
Now if your keel is iron, well... have fun. It probably isn't going to be easy.
Bear in mind if the keel was attached properly you could lose all of your keel bolts and the keel should stay on. The bolts are an extra layer of security. I was missing my three aft keel nuts and the keel was fine.
SV South Paw
2000 Hunter 340
South River, MD