The “Bible” (The Old Outboard Book) shows the original Champion spark plug as J12J,
and with a (1954) replacement as UJ12, which crosses over
to a modern Champion 592 spark plug.
It’s a 14 mm plug that has a reach of .46 inches.
Champion 592 Copper Plus Spark Plug RJ12C – 8 Pack | eBay
Most of these early motors ran with a plug gap of .025.
Hope this helps.
Prepare to be boarded!
