I went to prime my engine after rebuild and oil started leaking from the top area of the motor, I don't think its the head gaskets but i noticed this little plug circled in red in the picture. That is from my old engine but it is not on my new one, I assume oil is coming from there, any idea where I can get one of those or what size it is so I can get a new one, the old motor one wont come out.

Thanks

The picture is of the back of the motor, that plug is on the top deck surface.