Mth replacment for Lionel smoke unit fan motor?

It is my understanding that they both use the same smoke unit motor.

Part number? that I don't know.

OK all may not be  the same but some are interchangeable. I have bought them from our local store and used them

Both are not  the same.  They are not made by the same company.  Several part numbers exist.  Several differences such as location of screw mounting holes and length of the output shaft.  Some motors are longer than others.  In many situations they can be interchanged.  Lionel currently uses a different  smoke unit chamber that is made out of a plastic type material.  It has several mounting holes so you can use a motor with diagonal or horizontal mounting holes.    Lionel uses fan motors with both horizontal and diagonal mounting screw holes.  MTH uses only horizontal mounting holes.  Doing work for myself and Charles Ro, I always have a large stock of all motors.    The Lionel motors have the longer shaft.  I end up doing a very big number of smoke fan motors each month.  I am a fan of the Lionel plastic smoke unit as you never have the worry of the heater touching the metal bowl and setting up an issue.  Lionel uses this in top end locomotives and LionChief Plus.  It works well.  

Smoke fan motors unlike boards can be ordered by anyone.  Just contact Lionel or MTH and tell them what you need.  They are not a lot of money at retail prices.