You need to be more specific as to what kind of engine you're looking for, and what kind of build you're planning to do. Frame mount, friction drive, crappy China style or what. The 50cc 2 stroke like the ones on Harbor Freight's auger seem to be pretty popular and reliable. They can be had from several vendors for around $130.00. But you can get descent engines for $250.00 or less. I paid $220.00, there about for a Honda GX35 last year.
Good luck ! By the way, I think everyone will tell you to go with a Japanese brand if you can.