I agree with barnstuf. Stamp a repro plate and attach to firewall. When border agent asks to see VIN, show them the plate. It will match the paperwork, when they find a match right away, they won't be looking elsewhere and would have no idea there is a number on the engine. If the buyer is organized and prepared when he gets to the border, the agent is happy and gets everything done quickly. If you don't have your paperwork organized, they ask you a question and you stumble and can't come up with an answer, they open the flood gates and soon they bombard you with more and more questions and you are standing there slack jawed going, um, um, ahh, ahh, um.
When I import a vehicle, I get a 3 ring binder and put all documents in there, such as a printout of the online ad where you found the vehicle for sale, title, bill of sale, recall clearances, releases from USA customs, and an important one here is How did you pay for it? If any transaction was done in a form other than cash, such as paypal, money transfer, etc. The Canadian agents want to see the receipt. They want to make sure the receipt matches the bill of sale, so they can tell if the buyer is trying to pull a fast one by pretending they didn't pay as much for the vehicle to avoid import taxes. If you say you payed cash, they only have your word that the price on the bill of sale is correct. But if they find a paper trail saying a transfer came out of your account, then you're in trouble.
There was a story about a guy who tried that not too long ago, and the border agents figured it out and charged him something like 15 grand tax on a vehicle he payed about 10 for, but he was trying to tell them he payed 2. They have their ways of figuring things out, so best be honest. They are just tax collectors in a different uniform.