John, I have a patent on the spring loaded smoke unit, please cease and desist using my idea unless only used for your personal enjoyment. 
Here are pictures of a Lionel version in a brass J. MTH units have legs, short and long depending on the unit. So the legs may help with mounting. You do need to manufacture a stack to fit. If smoke can be mounted on the chassis with custom bracket that works too. That way shell comes off with smoke unit in place on chassis. I have done this for K-Line.
Lionel smoke units don't normally have legs. You can purchase some pre made stacks from Lionel.
In this case with a brass shell, the smoke unit had to be mounted in a confined location. I have used brackets, springs, wood dowel, all depends on the construction of the specific engine and constraints imposed. G
