There used to be a shop on Wua lai road, just north (the moat side) or the traffic lights. that sold small dirt bikes.
This place is closed now so not exactly a help! although the reason for me telling you is because i think they have another branch down on the road past the train station towards the super highway.
Not sure of the CC of the bikes, but they were selling for about 30 k if i remember rightly.
Also I saw a small dirt bike at Kawasaki the other day. Not sure if it was for sale though or just a spectacle.