First and foremost...that engine MUST be run on a mixture of no less than 24:1 fuel/oil mix..... I don't care what Billy Bob or Bubba tell you!
Second.....service the engine completely and you will have an engine that is just about bulletproof if you run it sensibly and take care of it. The water pump should be serviced. The gearcase oil should be checked, changed, and probably re-seal the gearcase. The ignition system should be checked, updated as necessary, and set. The carburetor should be cleaned and a new kit, including a new float, should be installed. From this point that engine is a real bear power wise and a pleasure to run.