They do little 50cc ones

http://cgi.ebay.co.uk/Petrol-off-Roa...d=p3286.c0.m14

My brothers have got offroad buggys, think they are around 400cc honda engine, they are wicked funm they have got 3 of them they picked up from ebay, you can buy them new too!

http://www.mudbugs.co.uk/warriorpge.htm, they have got the 13hp ones iirc!