While further inspection I noticed a crack in one of the walls of a small cavity.
Can if fill this oblong cavity with JB weld (or 3M 847) to seal the crack? I do not plan to adjust the wall.
aside from saving casting material the cavity likely serves no purpose so filling it with jbWeld after a good wire brush job and carb cleaner should not cause any problems .
I would not apply anything on the outer side edge of the main cavity in case a loose piece of JB weld end’s up the water tube or engine 5 miles from shore .
Ignore it, or break out the loose piece and ignore it. The cavity serves no purpose. Lots of them have gotten broken like that (from trapped water freezing)