I doubt that you'd have to become a Canadian citizen to practice anywhere in Canada. Many doctors come from South Africa each year to live and work in Canada. You may be required to spend some time in an underserved area for a couple years, but I don't see why Canada would deny a well-trained physician the right to practice. From what I understand, if you can complete the required exams and obtain a visa, you should be good to go.
I should mention that this is by no means straight from a a source, but I'd check out the Medical Council of Canada and College of Family Physicians of Canada or your intended province of practice websites.