I don’t know how many changes were made in the flywheel screws on the Early Johnson’s or, what sizes they may have been. The ones in my OA-55 & OA-60 are 1/4 X 24, another size that has gone out of fashion. There are other screws and bolts on these motors where the threads are now obsolete. The water drain screw in the pump motors come to mind. When working on these you want to keep track of all the fasteners as some are difficult to replace.
A "Boathouse Repair" is one thats done without having tools or the skills to do it properly.