Did you check the rings? As you have replaced the magneto and carb with those from a running motor, and you don’t see a problem with the reeds, I would suspect the rings are stuck. Take off the exhaust cover and move the pistons so you can see the rings. Take something like a small screwdriver and see if you can get some movement, pushing on the rings. I had a motor with 65 lbs compression. Like yours it wouldn’t do anything. The rings were stuck so hard I had to break them to get them out.
A "Boathouse Repair" is one thats done without having tools or the skills to do it properly.