I recently picked up a set of the Black Marzocchi 45mm Factory Series forks ... http://www.marzocchi.co.uk/moto/45legsend.html
They dont make them anymore, but I found a pair on Ebay.
I got them because my old riding buddy Terry (BAM Road Terry) bought a pair for his X-Country and said they fitted straight in. I had thought about using these a year ago, but dismissed it because I thought they were the wrong size for the stock triple clamps ... but I was wrong.!
Like other compatible Shiver transplants, this is probably the easiest high quality fork transplant for the X-Country / X-Challenge. They fit right in. All that needs to be done is to spin off the axle clamps from the stock forks, drill access for the compression adjustment and put them on the black anodised sliders on the Marzocchis.
Here is Terry's bike yesterday in the Hyperpro workshop in Alphen, Holland. Bas was respringing and revalving, and changing the weight of oil in the forks. The bike sits as high as an X-Challenge now