Hello
When it left the factory, it would have had 18mm plugs and unless someone's been mucking about, I suspect it still has.
The correct length for a bullnose Morris 11.9 or 13.9 (or even a later flatnose) have been unobtainable for years, but a compromise will do and its worth contacting the Greens spark plug Company:
www.gsparkplug.com
Regards
Alan