search this forum, several have posted in last year about keel bolts. job can be done at 'home' with big enough tools. there are sockets on the market for rounded off bolt heads, the members talked of pipe on end of handles to get leverage, etc. good luck.
My Mk 26 had already had the keel bolts re-done before I bought the boat. They left the orignal bolts and added 6 new Stainless Steel bolts next to the old ones. They look like 5/8th or 3/4". I don't know how long they are. If the orginals are leaking I think it would be best to pull the boat, let it dry for a week and then re-seal the keel to hull joint. You should be able to scrap out this joint and use a high strength epoxy filler suitable for under water applications. The boatyards will know what they are. I would not try to remove the orignal bolts. Just add additional new bolts in new drilled holes. You could do thid while the boat is still in the water. There are many prfessional compounds available for anchoring bolts in a drilled hole. Look up your local industrial fasteners supply house to discuss.