I'd scrap the boat. Cut the keel off for the lead content (you'll get your $300 back right there and then some), strip any hardware of value off the boat (spars, cushions, interior fittings, etc) and sell those, sell the trailer and outboard and apply the proceeds to your Macgregor.
Even if you repair it to use the boat do you honestly feel you'd be comfortable with the thought of "When will the keel fall off?" always in the back of your mind? If it were me it would ruin my joy on the water wondering "When?" while sailing.
Keels that move that aren't supposed to are never good things. I'd sooner troll the local yards looking for another SF22 hull that has been abandoned, make them an offer and make one good boat out of two than risk the loss of the keel at sea. You assume that even with your jacket on you'll get off safely. What if you're down below and she knocks flat as the keel heads for the bottom? You could easily be trapped or injured below in a hull heading for the bottom. Not to mention any potential costs if you need to remove the wreck afterwards.
Just not worth the risk to me unless you're really handy and feel confident in your repair. I was nervous just replacing keel nuts. Can't imagine doing the keel joint itself.
SV South Paw
2000 Hunter 340
South River, MD