You might contact the Cal 20 racing association. This is a common problem. If I remember correctly, I believe that the Cal 20's have a cast iron keel with a flange that is bolted with galv. iron bolts through the flange and bottom. With the keel dropped off the boat, the bolts are pretty easy to remove and replace (if I remember correctly, the heads are countersunk into the flange but are not welded or otherwise connected. If you use liquid wrench or PB Blaster you should be able to remove the nuts. This is a actually a very easy and routine repair job compared to the so called 'Catalina Method', which is necessary on boats that have the keel bolts cast into the ballast keel, and which is not a very good way to make the repair lasting.
As to gooping up the bolts with 5200, that is a bad idea. It will not help and may cause crevice corrosion and will make the ultimate repair much harder. I would suggest cleaning the bolts with naval jelly or ospho and then painting with a zinc rich epoxy primer. Because of the way that the Cal 20's keel is bolted on, you cannot undue the nut while the boat is in the water.