There are a few 250 cc push rod motors that you will have to compete with if you use the HF motor. Think Triumph, BSA, HD and some of the Chinese Honda clone motors. Besides lots of development, these all have gearboxes and clutches that can be strengthened for high speed use. $99 for the motor wouldn't touch the rest of the expenses to make that thing competitive. Try looking at some of the cheaper Chinese bikes and that may be your easiest entry point.