The 1942 service manual says your motor uses a P/N 190724 Champion M6 plug gapped at .025″ but I think they meant 6M and not M6. A modern replacement would be a Champion D14.
https://www.sparkplug-crossreference.com/convert/CHAMP_PN/6M
https://aomci.org/forums/topic/champion-6-m-cross-reference-or-replacement/
