Gas powered mountainboards suck.  I bought a 49cc motorized mountainboard off of ebay and it didnt even work.  I finally got it working, but even then it could hardly go anywhere but pavement.  The chain constantly slipped off, and the engine was a pain to get started.  If you go for a motorized board, stay away from the 4 wheeled pieces of junk that are being sold on ebay and across the web... you know, the ones that no one can tell if they are really 49cc or 36cc or something.  Go for a genuine wheelman or scarpar... now those things look fun.

The gas powered mountainboard was fun when it worked, but it didn't work as well as I thought it would.