Shamus,
I was fortunate in that someone had already installed a can motor in the Mantua 2-6-6-2 that I found. I am going to assume that the engine chassis are the same, and that the pilot, and trailing wheels are plastic. Also I am assuming that because the engine has electrical pickup,,lead engine/right hand,,rear engine/left hand,,that there is no electrical pickup from the tender.
I found that by replacing the pilot and trailing wheels with Northwest Short line wheels I got much better electrical performance. Where you have a tender, you have that many more wheels for contact.
The chassis of my loco has the can motor "RTV'd" in place, and uses the original rubber tubing to connect motor, and gearboxes.
I haven't had any problems with the loco yet, and have run it extensively on the modular layout.
I pulled the superstructure so you could see how this one is set up. I'll attach the pic.
Pete