I have done 3 mast step repairs. The mast grindes in the bottom and there will be a hole in the end. The thickness of the bottom is 4-5mm. From bottom and up to the deck it measures 14". So take a measure. In your case I would do a repair from the outside and remove the wooden donut. Grind off...
Well, I have discovered that 74-76 models are glued much better. Så I discovered a 76 model with a broken maststep. Then cut the maststep from this one, inserted and this gave me less trouble with things that are difficult to fix. Next level, I will fix the support wood under the cockpit floor.
Buy a Harken kicker and a MK2 sail for your own. Maybe a stiff mainsail sheet. Then you start working. It's like riding a bike. If you loose focus, you fail. It's an "all-in" boat. And the reward is much .
Do all boats have cubitainers? I have a 76 and a 97mod without. I also have a 75? , a 79 and an 80, a 2005 with..... I know 6boats are too much. I have to sell. But I got them for "an offer I could not resist."
Cover the mast with carwax, insert, twist turn 1cm, then see where the failure is. Sand away irregularities with sandpaper on a stick. Depth should be 14". Glue in a metal disk, lower it with a vacuum cleaner.