all sort of GPS trackers on the market... lojack has a motorcycle unit.. not sure if that would be usable on a bike or not, but likely could be run off a battery in a bag... get an underseat bag and put a battery in it and lowjack behind it, perhaps use the battery for a headlight too, no bad-guy would expect a gps behind the battery if there is a headlight up front.
i am seeing a good handful of results for GPS trackers online.