Ask a diver to help
I had to replace the swing keel cable on a Catalina 22 I owned a few years ago. I'm assuming there are a lot of similarities here. What I did was drop a piece of 10 guage stranded wire down the tube. At the bottom of the tube is a turning ball; a 1" Dia bronze ball with a groove around its circumference. So when you look down the tube you just see two dots of light. The wire goes down the aft side of the ball, since the swing keel is hinged forward. I snagged the wire with my boat hook and pulled the end of it up into the cockpit. I stripped an inch or so of insulation off and wrapped the strands of copper wire around one end of the new keel cable; the end that will get attached to the winch. Then a wrapped just enough blue tape around it to splice the two together. At this point I think I also tied another line to the cable, in case I had to retreive it. Next step: throw the new keel cable overboard! Carefully pull the wire back up thru the tube, followed by the keel cable. Now get a diver to attach the cable to the keel. I think in your case the procedure should be almost the same.
