@bob2 posted:Don't get too hung up on what shows inside the cab. These are primitive models, compared to the state of the art.
Good advise, that. I should have listened!
Not to leave well enough alone, I purchased this field-wound motor to give it a try:
Unlike the DC motor (with a 1" stack that came with this engine), this motor is only 5/8" and I thought it might "disappear" into the cab, so away I went.
The hardest thing to do was to remove the big brass coupling from the worm shaft: it's held in place with some kind of spring steel pins, and they were a real bear to pull out. Eventually I got it free and used a length of aquarium(?) tubing as a new coupling (you can see this in the photo.)
The mechanism came together nicely, checked out well on the bench; but when the time came to reassemble everything, the field coil was too far forward and too tall; it interfered with the boiler/cab. You can see in the picture where I trimmed a little of the bakelite(?) insulators, but it was a no-go after all. 
Strangely enough, when I put everything back the way had been, it seems to run better now; I think that the brass coupling now has the tiniest bit of "play" in it, so it's not as rigid as before and is a smoother fit. Is this even possible? 
Anyway, there you have it. I'm not going to re-finish/re-paint this model, as I really like the original patina, and it's in pretty good condition anyway. 
This has been a very enjoyable project; I wasn't sure if this would ever be anything more than a display piece, but thanks to all of you I have learned a lot about the mechanics of old O scale, and now have even more fondness for these old models...
Mark in Oregon
