They are the original thing, with a long and successful run. It will possibly have an uprated ride plate, intake scoop and impellor already fitted - few did not as the handling improvement is big over stock. If the engine has aftermarket pipes/other mods this can complicate things a little. The parts are getting a little harder to find off the shelf locally but any dealer should be able to find stuff for you and order it in - Kawasaki are quite good when it comes to parts for older models and this is an icon model that sold tens of thousands over the years..
First thing to get is a workshop manual. You can get lots of well meaning (and some not so) advice online, but the manual is the bible. No they are not free, check your local library.
For any marine engine sitting for that time this is a restoration, not just a tidy up. Assume the water pump impellor and carby needs service - the fuel tank flushing (no fuel cap!?) and that the engine is very dry internally. After dealing with the impellor and cleaning and/or rebuilding the carb take the spark plugs out and liberally inject two stroke oil into the cylinders, allow to soak and carefully rotate the engine by hand to coat the bores and introduce the lube into the bottom end - the manual shows how. The plugs should be replaced and you should not crank the engine over with no plugs fitted and the ignition and fill switch on - be careful of fuel leaks and watch for water leaks on first few times running. Make sure there is nothing in the drive impellor before cranking engine over.
