I got one of the IHC Old Timers for my son for Christmas (heard that before, huh) -- anyway, I managed to fit one of the little Lenz decoders behind the motor in the tender. It is only rated 1/2A, but it's been doing OK so far. Of course, I also had to add a light in that huge lantern, which meant a plug. I use 2mm spaced header/socket, so lots of connections are really easy in a small space (as in sound...) Make the connector one pair longer than necessary and cut off an interior pin (and fill its' corresponding hole) making a polarized connection that's difficult to plug in the wrong way.
Anyway, the thing works great, especially at slow speeds.