I'd start by researching all the relevant parts in the microfiche and around the net. Figure out what's the same and what's different. Things like the fork tubes and painted parts are hard because they'll have different part numbers for different colors of the same part. Stuff like bearings and axles will have the same number if they're the same type. That'll tell you what you need to change or adapt.
In addition to the obvious parts, check the rear subframe/fender/whatever in the back to make sure there's room for the new rear wheel.
For the painted parts you might find someone who has the bike(s) you're trying to match and let them drink your beer while you measure parts on their bike.
The companies that sell aftermarket suspension for the X bikes might be able to help too. A lot of people don't like the XC suspension, so going straight to aftermarket might be the way to go. It's probably cheaper than new OEM prices anyway.