I have embarked on the next addition to the W&A stable of locomotives, the Catoosa. She was a Norris built locomotive, same manufacturer as the Yonah. As always, a key reference for the Mantua General alterations is Al Mueller's manual...
I use a motor from a fella named "Motorman" who restores and rebuilds a wide variety of motors and can be found on the internet. He operates via eBay usually. This motor is a Swiss Maxon 13 mm 7 pole coreless motor-High torque (150731330992). They run about $45. My locos are able to pull 12-18 car trains which is quite a feat given that the Mantua General's are not known for their pulling power. Of course I weigh my cars below the NMRA standard to about 30 grams.
One of the more prominent elements to his rebuild is his idea on making room in the tender for a new motor, decoder, speaker and capacitor. Once the tender top is removed, grinding away some of the frame is needed to accommodate these parts. And since the original model comes with a plastic wood load, that is not acceptable for a fine scale model replication. Here's a page from Al's manual on preparing the form for the wood load.
Now here's the result of my attempt to replicate his instruction. You can see that I added a brass plate and water tank cap; the cap is from Precision Scale.
These next two photos show the tender interior exposed. By being able to remove the wood load there is easier access to the motor section without needing to remove the entire tender. It also allows for more ventilation. The black tip coming out of the tender front is a drive shat cup from NWSL. The drive shat mechanism is another redesign element also offered in Al's manual.
The decoder is a Tsunami Micro. But these have been discontinued. Their replacement decoder is called Econami. Al has installed one or two and vouches for their reliability and easier programming as well as installation; these are slightly smaller believe it or not.