Just swapped out the radiator fan motor on my 93 MTX. Took the old motor out of the fan shroud (which was rusted in there REAL nice!). Had to take a hammer to the back part where the nub sticks out and used some PB Blaster in the process. Be careful though, because the whole shroud assembly is plastic!
One thing to note is that the fan motors are in fact different for the MTXs and the ATXs (as well as in price). The MTX fan motor cost just over $60 at Advance Auto. Also, the MTX fan motor does not include an electrical connector, so the old wires and connector need to be cut and spliced on to the new motor's wires. However, the ATX motor does have the connector.
Hope this helps!
-Mike