I had hull 877 ( a 1965 or ’66 ) – the older boats are built very stout ! I would seal the (2) holes with fiberglass and top with gelcoat, re-drill for the new boom crutch configuration ( they sell a kit ). It’s a better design.
If you’re hell bent on the old config, then go to hardware store or home depot and buy brass tubing ( are you sure it;s not cooper ? The through hull seat and hull frains are copper ). You can buy a flange kit that puts a flange on one side of the tube, cut to the right length then epoxy in place.