Remove the plug without distorting it too badly and measure it then go to an auto parts store and get a replacement. Dorman makes all sizes plugs.
If you cant find a replacement metal one, use an expanding rubber one that has the nut on it that you tighten. A very thin skin of silicone sealant on the edge to insure the seal.
When replacing a metal one, be sure the hole is clean and dry. I use a piece of fine emery cloth to clean the block surface of the hole and a very thin (very thin) coating of Indian Head gasket shellac as a sealer. Use a socket that fits the ID of the plug and an extension to tap the new plug into place
I've replaced dozens of these on various engines this way over the years and never had one leak.