I don't know about riding across a boarder - he/she might object to having a bike rode across them. :mrgreen:
Seriously, if you are bringing the bike into the USA for riding, off-road or road, then I don't think there is much to do. I don't think it is any different than any other vehicle. You might be asked if you are going to sell it and they might require you to bring it back to Canada. Not sure. Personally I have always gone the other way - into Canada and back - on a street bike, with no problems or questions about the bike.
If you are wanting to register/license it here for the street, then if it was made after about 1982 you will probably need it to have an EPA/FMVSS cert/sticker for some states - others won't care. If you are trying to sell it, then there are other issues.