I think the Christini 2WD bike did really well in the Endurocross the past few years. The kit costs abot $3K I think, but if you race it may be worth it.
I'm surprised more people don't do it. I see guys spending hundreds of dollars on plastic and decals, and multiple times that just to gain a couple of HP. You'd think AWD for 3 grand would be tolerable.
For me, $3k would bike a very nice second bike, so I just couldn't justify it. It would be awesome to run one for a weekend though.
Joe