The book I have list the Champion 5M-J superseded
by the D9-J which now crosses with several plugs
the Champion being D9. None of these plugs are
familiar to me nor is the motor your working on but
I suspect it needs a short plug. I don’t know if the D9
is a short plug. I’ve never seen any spark plugs that
didn’t have a part number on them somewhere.
Tubs.
A "Boathouse Repair" is one thats done without having tools or the skills to do it properly.