Hi There My Three Simars are all 18 mm. Champion no8 com was a very popular plug at the time you should be to get an alternative try the green spark plug company they might have something

I use NGK A6 or Champion D16 or similar spark plugs. I have owners manual and parts list available as pdf files. I am also custodian of the factory records and can date machine from serial number 35????. Parts are not available although it may be possible to find some secondhand parts from machines broken for parts. Bearings and seals maybe obtainable from good suppliers. Do you need any specific parts?
Some of the original engineering drawings are held in the archives at Museum of English Rural Life (MERL) Reading. I have been helping catalogue them, until Covid19 put a stop to visiting the museum.