what mods can be made bought for the kawasaki zx9 engine?

Same as any other engine,depends how deep your pockets are.
Try here http://www.zx9-r.net/nav.htm
Holeshot racing and TTS do big bore kits etc for the ZX9. And of course there is Chris Allanson at Z cars who pioneered the twin engine installation.

ATB

Alan.