In the last few years I was with Motor Coils, we started to rebuild some narrow gauge motors, mostly D29 and D31, along with a few GE 761's if I recall correctly. Its been over six years since I left for new career challenges, so the memories are getting a bit faded, but from what I recall, D29 traction motors were, as stated above, simply EMD's narrow gauge motor. An analogy would be that the D29 was EMD's narrow gauge version of a D77, although the armature windings and field coils were vastly different. To continue the analogy, a D31 motor bears a resemblence to a D87 in the fields, although the coils are much smaller, and as others stated, the armatures are interchangable, although there are insulation and bearing upgrades in the D31 from the D29, just as there are often only insulation and bearing upgrades between the standard gauge EMD traction motors, such as a D67 to a D77.
In both, the windings are much smaller, both in size of the finished coil and the conductor, with more turns per field coil than a D77 or D87. Armatures are of a different coil design, without equalizer coils in the windings.
Don't really recall much more than that, you could check the sites of GE, EMD themselves, or Swiger Coil Systems.