Re: Big OOPS 1961 SEAKING 25 HP
This is probably too late (a few months) but I did this yesterday to my Gale 1961 25hp Sea King. Instead of the drain plug I took out a smaller phillips screw. It drained the oil but the screw wouldn't go back in. The correct answer to you question is probably much more complicated than what I did. I stuck a really small phillips in the hole and played with the fwd/rvs shifter until I got the screwdriver to go "all the way" in (about half an inch). I thought that if I could get the screwdriver in the hole, then the screw should go in there too. After several failed attempts at getting the screw back in the same hole with the same combinations of playing with the shifter, I filed the end of the screw down to a point (like the phillips had) and it wiggled right in.
I am by no means a marine transmission expert, and I probably messed something up by doing it this way, but it worked.