I use the MTH smoke units in many upgrades, 3rd Rail, Weaver, Williams, etc. I've used them in K-Line and Lionel to replace puffers as well.
If you're not installing PS2 or PS/3, the command smoke unit will require a resistor change for use, I use anything from a 20 ohm to a 27 ohm resistor. You also need a source of 5VDC power for the smoke fan. Of course, for steamers I'm normally installing a Super-Chuffer II, so I have the fan control all handled.
Here's the PS/2 unit in a K-Line.
Here's the PS/1 unit, this will run directly off track power.