A little further North of Larsen's is Skipper Buds in Winthrop Harbor, but the yard that does the most work in all of Chicago Land is on the South Side. Crowley's is by far the biggest and does the most work of the others mentioned.
Some people don't like them but many say good things about them. They are on the Calument River and 95th Street. Can you get the boat down to them, is it trailerable? I am sure they would be able to do the work you require.
Courtney is My Hero
If a man is to be obsessed by something, I suppose a boat is as good as anything, perhaps a bit better than most - E.B. White