A 250cc 4 stroke is what you need. Your choices are: Honda XR 250, Honda CRF 250, Kawasaki KX 250F, Suzuki RM 250F, Suzuki DRZ 250, Yamaha TTR 250, Yamaha WR 250F, Yamaha WR 250R, and Yamaha YZ 250F. Look for them in your local classifieds, auto & cycle trader magazines, ebay, and craigslist.