I swear by Densos these days. Equivalents of the J6C is W20S-U or W22S-U. I run these in both my 1963 10 hp sportwin and in my 6 hp Fisherman and I’ve found that no other spark plug works this well in my engines.
The 1962 engine has a thermostat, so the "fun" thing to know about NGK plugs is that their "heat numbers" run in the opposite direction from Champion, so to replace a J4C, use B8S (not B4S!!)
I’m going to run it very soon. New roof, rain during new roof, wanting to finish the ’62’ stopped me from running it. I want to run it before I break in the ’62’.