OK. If the RS2 you are modeling has factory installed steam generation, the parts you need are made by Custom Finishing. Check the Walthers website under manufacturers. The parts are listed. The stack is part 247-151. You will also need a mushroom vent part 247-206.
Some units had a different stack arrangement and may use part 247-207.
The first stack was mounted next to the cab, the other near the end of the short hood. Both used the mushroom vent.