Hi, this is Russ,! I will give you the benefit of 28 years experience and teaching. The part is called a FREEZE PLUG, they are supposed to pop out if conditions cause the coolant to freeze in the engine so as to not crack the block. The are sold by size in 64ths of an inch. The Generac dealer will have them as do auto parts stores. The size is stamped into the plug also. Unless you have the specialized tools to install them, it is not wise to try doing it yourself, you can cause major damage!
THANKS AND GOD BLESS!