I have a couple of these old Champions, one of them my Dad bought new. I’ve been using Champion D16 spark plugs with no problems. There are lots of 4.2 HP parts motors here in Minnesota, as they were made here and a zillion of them were sold. Up until somewhere inthe mid 1980s, I could buy factory parts from Swanson Outboard which was a 20 minute drive from me. Lyle Swanson got the leftover Champion factory stock when they went out of business. Unfortunately, Swanson Outboard is long gone now.
Dave