Stop right there. In the first place, you cannot test a water pump by running on muffs. The pump is not pumping when you are doing that, only allowing hose water to pass through. Besides, muffs do not work on that motor anyway.
Get it on a boat, on the lake and see if it works. And if it doesn't, shut it off and don't be letting it get that hot. Depending on how hot you let it get, you may have already damaged it.
I would have replaced that impeller, even though it looked good. Seen LOTS of those "looking good" impellers shed their vanes.