I will block the nipple with the red arrow with a vacuum cap or take out nipple and put a screw in or solder it closed{content}
Looks like you could unscrew the red nipple and replace with a brass plug, then you never have to worry about it leaking.
1954 Evinrude 7.5 HP
1954 Evinrude 15 HP
1954 Gale 12 HP
1956 Gale 12 HP
1956 Evinrude 5.5 HP
1960 Evinrude 18 HP
1973 Johnson 6 HP