Ransomes spark plugs

Hi All, am a new guy so please if this has been bought up befor, excuse me. I have aquired a Ransomes Minor Mark 6 Motor Mower, I believe a 1951 model and was wondering where I might get a KLG ML 3o Spark Plug or the equivalent [I think] Champion D16, D21 , NGK 3020...are these right or are there others. I look forward to your advise, cheers Robert