You are no more liable for the theft by having recalls fixed; however, you were liable from the day you became the owner of the machine. To get a bearing on your sled's history, go to http://www.purepolaris.com/en-us/Sno...earch-VIN.aspx
and enter your sled's VIN. The website will list the last known owner's info, and you can proceed from there in contacting them. The British Columbian snowmobile registrar also likely keeps a list of reported stolen sleds. You may find yourself in a difficult situation; the best course of action I can recommend is just to be level and above board about the whole thing.
As far as the updates, you really have to get them done, particularly the ECU reflash, to avoid burning down the motor. You cannot, and do not want to, get around a lean-running condition from poor fuel map calibration, and other major reliability issues.