Please get something more than 50cc's... My lawnmower has more power than that.
Take a looker at one of these:
http://www.yamaha-motor.com/sport/pr...96/0/home.aspx
Or these:
http://powersports.honda.com/2009/crf230m.aspx
Or these:
http://www.kawasaki.com/Products/Pro...scid=14&id=417
The Aprilia site is down, but they make dual sports/ crossovers as well. An aprilia would be much easier to get your hands on seeing as how they are Italian, not American or Japanese.