The other screw, if it hasn’t been lost, should look like this. The end is meant to be in the grove on the main bearing to keep the magneto plate from lifting. Notice how mine has two lock washers to keep it from going in too far, and locking up the magneto plate, but still in far enough that magneto plate it can’t move up.
A "Boathouse Repair" is one thats done without having tools or the skills to do it properly.