I have two Rivarossi steam engines, NYC Hudson and PRR Y6b. I've owned both new since my childhood. I've replaced motors on both (found on Ebay). I've checked at a couple of my LHS's (Trainworld and Nassau Hobby), haven't had any luck at either. Seems that when Rivarossi went under much of the documentation was lost. Parts are on the second hand market gotta hunt around.
Great loco's for sure.