Looks like you are pointing at a Welch Plug. But are you trying to indicate the casting?
Under edit. the second picture just came up. A Welch Plug should have filled that hole. They are available from most good auto parts houses and I'd assume google could be your friend. You'll need to measure the I.D. or if the original came out in one piece, you can measure that.
I don't think they'd put a breather right below the oil fill.. I think you need one of these = Amazon.com : welch plug assortment , but check the manual and see what (if) they say?
Here are a couple of pics from when I replaced the governor gear in the M18 residing in my 682. The first pic is right after I split the block and found just the bare governor shaft. Second pic is from the blown up KT17 that was originally in my 682 showing the intact original governor gear..
Oil blows out of it to fast to tell about the idling. tried a 3/4" softplug but it was to big. prolly metric and just bought a rubber plug to test things.