JR is right, it ends up being a bigger headache and hassel then to just fix up a nics SJ or other ski.
I put a 750 SS into my 91 650 SX some years back. The swap on that ski was pretty simple.
Here is what you will need to do to make it fit a 440 or a 550:
1.) 440/550 Bed Plate
2.) 650/750/800 Engine to 440/550 Bed Plate Conversion Plate (eBay)
3.) 650/750/800 Engine (Cases, Crank, Cylinder, Head, Intake Manifold, Carbs, F/As, ect...
4.) 650 Kawasaki Exhaust Manifold
5.) 650 Kawasaki Exhaust Pipe (Smaller The better!, it needs to fit in the ski! the stock one is pretty tiny. So is the PJS, Mariner, and Kerker. The Coffman and Factory pipe are probably waytoo big
6.) 650 or 750 Electrics (Flywheel, stator, CDI, Voltage Regulator, ect...)
7.) If you use 650 electrics, you will have to have the cases drilled and tapped for the 650 stator mounting screws and have a hole drilled through the cases for the cable to pass through them. This is stock on a 650. The 750 and 800 engines have the bosses cast into the cases where this needs to be done. Search around, this topics has been covered many times.
8.) Use the water box that came in the original hull.
9.) I am unsure of the drive shaft issues with your ski/hull in my 650 I just used the 650 shaft
It really isn't that hard to do a 750/800 into a 650. On the 440/550 there is a little bit more work to do. Things will be difintely cramped in the engine compartment too. I am unsure what you need to do about the drive shaft support, I never owned a 440/550. I do know that people swaped pumps and drive shafts from different years because some were better.
Also keep in mind, no matter how much power you have, if you cannot get it to the water, you won't be going that far out of the water. I expected my ski to jump ten feet out the water yet is barely got 2 feet on flat water.
Still though, I am going to put an 800 SXR engine in my X2 when I get the funds....
DAVE