HI,
Here is a link that may help you....
Paired spoke vs radial/normal lacing - RoadBikeReview Forums
Personally, with the sustained high speeds you can achieve using a motorized bike and the fact that most roads are far from perfect, I personally would lean towards a more traditional and heavy duty wheel....I don't recall seeing too many paired spoke laced moto cross bikes out there! ;-)
Good Luck on your build....Ride safely!
Andrew