There is a place for the line under the sink, right?
One way to replace it is with the boat in the water with the CB down partially at least. Get the line down through the tube for it into the water and dive and reattach it. Some people are going with lines now.
Ours has a SS cable held on with a crimped fitting on the end of the cable.
http://purplesagetradingpost.com/sumner/macgregor2/outside-40.html
I'm guessing it came from the factory like that (or maybe not as that is an aftermarket centerboard). Under the sink the SS cable changes to a line just before the first turning block if I remember right (not home now trying to cross back over to FL from the Bahamas).
It goes through two turning blocks and comes out on the right side of the sink just above where the cleat is in the picture above. I added the block above the cleat and a couple more and ......
... now the line comes out in the cockpit and cleats off there. I didn't like having to go below to raise and lower the centerboard. More here...
http://purplesagetradingpost.com/sumner/macgregor2/rigging-19.html
Besides putting it in the water another option would be to ....
.... make a platform for the transom.
http://purplesagetradingpost.com/sumner/macgregor/outside-3.html
Lower the tongue of the trailer to the ground. That will raise the back of the trailer and the boat's stern. Make a platform the right height to support the transom at its new elevated height. Jack the tongue up and the back of the trailer will go down and you might be able to lower the centerboard enough to reattach a cable or line (could also at this point remove the wheels/tires to lower the back of the trailer more). Do all of this very carefully as that boat could kill you if it fell on you).
Sumner
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1300 miles to The Bahamas and Back in the Mac...
Endeavour 37 Mods...
MacGregor 26-S Mods...
Mac Trips to Utah, Idaho, Wyoming, Canada, Florida, Bahamas