Mounting a ZX9 engine into a ZXR750 frame is a conversion with a long history; performed numerous times over the years by amateurs at home and professional mechanics in race team workshops around the world.

Here's is a list of online resources which I scoured through to gather as much information as I could, before attempting the conversion myself.

http://homepage.ntlworld.com/webzxr/zxr750r/Features/91zxr7-9.html
http://home.swipnet.se/Kwackerjack/