The transmission output shaft is metric - 16mm dia with a 5mm keyway.
(as taken from the transmission manual found here:
http://www.ifirobotics.com/kitbot.shtml in the output shaft drawing spec)
Paul, please correct me if I'm wrong.
Teams can buy any size sprocket they want (with a bore less than 16mm), rebore it to 16mm and rebroach to 5mm. There is probably a fancy way to broach this keyway, but the simplest way is just to buy a metric broach set.
Anyone else have advice?