The BME 50 isn't a legal engine for racing. Any stock gas or glow engine up to 35cc with stock muffler meeting AMA noise requirements. Any prop except on 35cc glow like the Moki/Mark 2.1, which is limited to an APC 17x12 pattern.
See the new Sundowner rules here.
http://www.usrainfo.org/sundowner.htm
Hope to see you there.
Mike