Conversion Guide for '91 to '95 ZXR750/9R
Based on Pete Pham's document "Instruction for installing a zx9 motor in a zx7"
Pictures kindly provided by Kai and Stahli from    (Well worth visiting even if you don't speak German)

You need an engine from a '94-'97 ZX9R. Post '97 ZX9R engines will not fit as the engine mounts are very different! The wiring harness of the ZX9R is pretty much identical to that of the 750, but you do need the ignition box of the ZX9R as the 900 motor does not rev as high as the 750 (hence you could blow it up if you use the 750 box). You don't need the coils and you can run either the ZX9 or the ZXR750 carbs and exhaust. The ZX9R carbs should give more top end whilst the carbs off the 750 should give better midrange.
If installing the engine in a '91/'92 J model bear in mind that the airbox is very different to that of the ZX9R or of the ZXR750 L/M and it is not a ram air box, hence if you use the ZX9R carbs they will need to be totally rejetted. If you are installing into a K or M model I would suggest you keep the FCR carbs that these bikes come equipped with.

Pull your old engine out taking care to label every cable and connector, taking notes on the order you removed each bit. It is a good idea to label all the electrical connections so you can easily put things back afterwards. Make sure you note down the routing of all cables.
Install the new motor using the lower and middle mounts but leave the nuts loose.
On the left side of the motor is an oil return line. It ends up hitting the frame a little so you will need to use the dremel to grind away some of the metal on the frame to clear the line. Very little needs to be removed here. An alternative may be to use the oil plate that comes as part of the BRG conversion kit (the rest of the kit is only needed if trying to fit a 9R engine into a '96 on ZX7R). (Click on the pictures to enlarge)
You will notice that the upper mounts do not line up. This is because the stroke on the 9's motor is 10mm taller. Hence the hole in the frame and the hole in the motor line up like a figure of 8. (Click on the pictures to enlarge)
Use a 1/2" drill bit in the metal adjuster sleeve on the frame as a guide to clearance the hole on the motor. Take care to just drill out as much as it is needed to get the bolt through and no more. (Click on the pictures to enlarge)
On the motor, use a dremel tool and grind out the channel that holds the nut on the upper mounts so that the nut will line up with the new location of the bolt.
Once you are finished lining up the bolt and nut, tighten up all the mounting bolts starting with the lower back bolt and work your way up. This will ensure proper alignment of the motor and chain and rear wheel.
Reinstall the exhaust, your original radiator and hook up all the water lines, chain, and electricals. The ZXR's thermostat will work, but it may be a good idea to install the ZX9R thermostat as the 900 engine is supposed to operate at a higher temperature.
Again, make sure to use the CDI box from the ZX9 instead of the original one from the 750. Reason for this is because the ZX9R has a lower red-line than the ZXR750 and this is controlled by the box. Some people have tried using a 750 box to get more power through letting the 900 engine rev higher. This might work, but it also takes the engine beyond its safe rev limit and is likely to blow the engine hence not to be recommended.
On '93/95 bikes the original airbox will fit, though depending on the carbs you use you may need the carb-to-airbox adapter plate from the ZX9R. The '91/92(J/K) airbox will fit, but is also likely to require a fair amount of modification to flow enough air for the 900cc motor. Some converted J/K models run without box and filter, but the idea of running without air filter frankly scares me.
The zx9 motor should also come with a water reservoir on the side. If you have a pre-'95 that has the water reservoir on the tail, you can use the new one. You will just need to trim the original black plastic cover on the side to clear the new tank.
Go get it tuned and dynoed. With the carbs jetted and a full system, you should have about 130-135hp at the rear wheels.

With Thanks to Pete for providing much of the information above and Kai and Stahli for the pictures