Doen't matter if a OB is well flushed with fresh water after use, with humdreds of hot/cold cycles over the years will accumulate thin salt layers even crusts inside all water passages which will inevitably lessen the capacity to disipate water properly while runniing inside powerhead's water passages.
Powerhead will work at a much higher temp than if having impeccable clean water passages as when OB was out of the box, changing thermo, flush valve, spring won't do a thing, waste of time and money if that issue is already happening.
If it's an OB that lives on back transom while moored which never been flushed with fresh water after use, much worse. Try flushing OB at fast idle speed with Salt Away Product on a barrel, hope product performs as stated.
Happy Boating