The pre 98 zx9r engine is almost a direct swap into the pre 96 zxr 750.
The also fit into the 96 + zx7r but require a little bit of extra work.
As Strika said Groggles has a 95 / 96 zx9r engine in his 95 zxr frame...
The 98 + zx9r engine is wider and doesn't fit as easily but can be done if you have the time. There was one floating around here a while back... Ben from extreme creations had I believe.?
Heres some links
http://homepage.ntlworld.com/webzxr/zxr ... links.html" target="_blank
http://homepage.ntlworld.com/webzxr/zxr750r/" target="_blank