Everything I have read about doing this says it would test the patience of a saint and many pro bike machanics - four internal linear roller bearing races to manage. The Cannondale website used to have a list of approved Headshock service centers.
I had one rebuilt by these guys about six years ago, they're in Germany. It was not cheap but they did a good job. At the time I just thought it was the easiest option, retirement means that it wouldn't be an available option anymore!