looking for a engine for a 1980 honda XR250 dirtbike

i have a 1980 honda XR250 dirtbike and the engine has worn out, mainly the top end. i am looking for either a complete running engine or one that has good top end parts, or parts so i can fix mine. i need the cyl, piston & rings, timing chain, tensioners, gasket set. i haven't found any place that sells parts for this old bike anywhere, if you have these list it so i can bid.
thanks