I don't think I have ever seen a threaded coolant plug on a YM2210 block. Many of the 3 cylinder Yanmars have them but, for some reason, most of the 2 cylinders don't. The freeze plug holes are typically too shallow for a block heater. I have seen some freeze plug extension tubes on ebay (for Kubota I think) but I'm just not sure if I would trust them to stay in there. I'm sure they must work but looks suspicious to me.
Many years ago I made a few batches of water pump back plates that had a small block heater threaded into it but that block heater was discontinued and we could not ever find another one thin enough to screw into that plate.
(edit: I found a picture of the custom water pump back plate in case anyone was interested)
Aaron