Decals .. several sources are listed on the “classified” page of your Antique Outboarder Magazine. Chose water slide or vinyl stick- on.
As for Spark Plugs, always many at club swap & sell meets. Or search e- bay for the more exotic ones. Just general info, the modern Champion plugs are longer than the older classic ones, so just because the numbers are the same you can spot the difference right away. I don’t know about other brands but many ( most? ) old outboards used Champion plugs.
Joe B