clutch problems


I have bought a sportrak for spares, engine had been damaged replacement was sat in the engine bay so decided to try and put it back together and put it on the road. When I tired to fit engine in finally realised that the clutch plate did not fit on the gearbox shaft. Have purchased a second hand clutch plate and it seems to be the same size as the one I had. Don't know what vehicle the engine came from but looks the same as my other sportrak. Could anyone tell or help me with why or should the clutches be different sizes.