Here y'go:
http://www.armbell.com/kz400/index.php?sid=bc8f3f6ab7004bd704129f419b1f27ad&mforum=kz400It's an outstanding forum dedicated to the vintage Kawasaki twins.  All the help and info you'll ever need, especially including the 'home' site kz400.com
Kawasaki had two twins in the mid-70s to mid-80s:  a 400 that grew to a 440, and a 750.  It'll be one or the other.
The KZ400 was Kawasaki's version of a Honda CB350.  It was Kawasaki's second 4-stroke, following the KZ900 of myth, legend and immortality.  The 400/440 is a great bike.  Strong enough, good handling, counterbalanced motor, very 'nice' bikes.
The 750 is most of all that, too, but not as numerous.  A bit on the heavy side.
Enjoy it, whatever it turns out to be.
Kirk