new atlas motors N scale

I run primarily Atlas engines. I have some that have a new, slower running motor. How do I replace the older motors?

Most of my engines are either Kato or Atlas. Rather than changing the motors, I usually run a lashup made up only of the older ones. I know this will be a bit odd but changing motors is likely to be expensive for me!
Cheers,
Cliff
http://cliffordconceicao3310.fotopic.net/c328807.html

A couple of questions. I have not replaced my motors yet, but I have some sitting on the shelf waiting to be installed in RSD4/5s, along with decoders. Most N scale motors simply get inserted in the metal locomotive frame and have driveshafts that fit into the flywheels on both ends. There is nothing that fastens them in place except their shape and the shafts.

What model Atlases are you going to be putting these in? Are you stuck, or is this a pre-emptive question?