Hi! I bought a Kawasaki gpx 750r 1989. It was not used since 1997. I wonder, should i change the piston rings?or any other mechanical part?except for fluids an so...thanks :)

No, their would be no reason to change the piston and rings first up, unless you know that they are worn from the previous owner, Get the bike up and running first and go from their, will be a good chance the piston and rings will be fine.

Change the oil first up as their is a good chance condensation has occurred and put water in to the oil. Drain the fuel from both the tank and carburettor, as this would have gone stale. Change battery or jump start if required.

suspect. With the spark plugs out, crank the engine over a few times. (ensure oil level correct) If it winds over all good, put a few drops off oil in the cylinders. This will help build compression. Re-fit spark plugs, Full choke and give it a shot. Away it should go! Aerostart or ether may come in handy.

Goodluck!