I don't know where you can find the part you need, but my first bike was a CCM Imperial Mark IV with 20 in wheels. Red with white trim. I was 8, so it would have been '61.

Good luck finding what you need. If it's just the arm, it shouldn't be too hard to find a suitable one. As I recall, those Bendix coaster brakes were pretty common, as advertised in millions of comic books.