well its a half truth about the motor, the basic design is the same, except there are minor changes as the models progress. One minor (but still the biggest diff ive seen yet) is the oil filler hole is positioned differently, and the same goes for the clutch actuator on the engine. Aside from that, and minor "make it look tidier" enhancements, its basically the same.
Heck you might even be able to fit later model peices onto the early model bike and get a better designed bit of machinery. But still, always look it up, places like
http://www.bikebandit.com and
http://www.buykawasaki.com have the microfiche's online so you can compare part numers, and even to a certain degree, appearance of different parts.