Yes, that is an 18 mm plug, which makes it more expensive to replace, since they are obsolete these days. I think they can be obtained, at a price, and perhaps with a limited range of types. Don't forget to reinstall the copper washer between the plug body and the cylinder head, or it will leak and erode both plug and head. Incidentally that plug has a stub side electrode instead of an adjustable one, so you can't adjust the gap. I can only recall seeing that before on aircraft engine plugs, which had multiple stubs right around the perimeter.
The conventional way to "stamp" markings onto cylindrical metal objects is to roll it on: the information is set up in metal type as if it were going to be stamped, but the type is moved across the cylinder, while the cylinder rotates underneath it.
