I don't think you can fly a Canadian registered aircraft in Canada
without a Canadian certificate. I believe you can get a Canadian
certificate on a temporary bases on your U.S. but you have to visit
their FSDO.
I would keep the existing registration and get a letter from the old
owner saying you are authorized to fly it in Canada. I would avoid
letting the customs people in the U.S. from knowing you bought it
because I believe there is a big import tax.

-Robert