I have a 1979 johnson 100hp. I just changed the powerhead to a 79 115. everything works great after the swap. I need to change the water pum/impeller being that it has never been changed in the 27 years that I have owned this boat... I know.. I know... (It still pumps water great!!!) My question is: Do I change the whole pump, or just the impeller? Thanks for the advice.