Most common name is a body plug if its on a car ( they have heaps of them ) but I dunno what the outboard peeps would call it . You should be able to get one from an auto parts shop for a couple of bucks .
I'm not at all sure that this is the original plug. At least on my pre 1972 outboards there is a rectangular little metal plate, which is the same color as the rest of the lower unit, that is held in place by 2 screws. It is called a "cover plate"
