When you make your way down to the USA, before crossing over, make a stop at Canada Customs. Go inside and ask a officer at the counter for a Form Y38 (Identification of Articles for Temporary Exportation)
(aka the green card)
They may or may not come out and look at the bike, so make sure you have it with you. If it has no serial #, they may affix a tag onto it with one.
Then whenever you get questioned for proof of owernship when returning to Canada, show the card. Carry a bill of sale with you as well for secondary documentation if needed. If it's a private sale, just get the seller to hand write one up.