Jon:
Miniatures By Eric makes an N Scale SW1200RS shell. For some reason they are not on his website but I have seen them at Central Hobbies and a friend is building one. There is a version for CN and CP and I belive they include the Flexicoil trucks and the spark arrestors on top. I would email Eric and ask, or contact Trains & Such in Calgary to see if they can get one for you.
Tim