Just perusing ebay, check out this auction from Mr. Bob Hannon:
It's for newly wound Ives Armatures. I remember speaking with Jim Cohen many times about armatures (or lack thereof) and Jim couldn't say enough good things about Bob. I have some of Bob's armatures in a few of my Jim Cohen trains and they are as good as anything from the Ives or Lionel factory.
There are quite a few things that can go wrong with an original standard gauge engine. More often than not the key item that effects performance and is hardest to replace IS the armature. Winding these things is almost an art form. Not trivial by any means. It can be done by a dedicated hobbyist however I've seen Bob's work and it is stellar. Read is his return policy on eBay and you'll get a good sense of how ethical he is.