Noting you have it running now, it might still be worth considering cleaning
system with SaltAway or similar. You know the history
of your engine, and whether it has run enough to salt
I had a similar problem on a Mariner 15 HP, and when I stripped it down found the cooling
path around the head
was completely salted up, and the cylinder channels were heavily salted. Cooling water
was reaching the telltale via a relatively narrow, salt
encrusted channel next to the cylinders. So despite new impeller, the cylinders still overheated.