No speed when throttled up

   Last month I blew though an impeller on my 1977 Ski Tique. The paddles to the impeller stayed together but the axel broke away from the rubber housing. The engine did not heat too high for too long. The impeller was replaced and was functioning well. During the replacement I did fine bits of a previous impeller breakdown.
   Since then I have lost the top end. It runs fine from 0-30 mph. Once I pushed it to full throttle, it hesitates and would not go over ~32 mph, temp climbed ~180 and I got a bit of a burning smell. I throttled back the temp dropped below 140, smell went away and the engine ran fine. It runs fine up to that mph. Hmmm.

Ideas on what it could be?