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.
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..