You're right, every time I look at that loop, I want to plug it. Just lazy I guess. I think those are 3/8 pipe thread plugs, not metric. I believe the Yanmar part number on a 2GM20F is 23871-030000 (same for both), but a Yanmar dealer can confirm.
Go here for a parts breakdown: http://tehstar.ru/assets/files/manuals/2GM20.pdf
It's on figure 25.