I'm looking at buying a '87 Thundercraft Citation 170. Overall condition is pretty good. In the ad. it states that it "needs the impeller replaced", otherwise it runs fine. How big of a deal is changing the impeller? It also states that "it's a newer Mercruiser Alpha 1". Is this a "take it to a shop" repair? Or can I do it myself? I have a pretty good grasp on engine's & stuff. As you can tell I'm a first timer in the boat world, so take it easy on me here. Thanks for the help