The Johnson/Mabuchi silver cans that have been used by Tamiya over the years have changed spec a title from time to time, but became "slightly more" consistent over the past 10 years. If I'm not mistaken, the older, shiny motors were Mabuchis and often had 4 slots in the can, rather than two. These are faster. Among the newer motors, the Johnsons with "cs" stamped on the brushes are supposed to be quite good.

There are loads of videos on YouTube showing basic break in methods. One of the more effective ones is to run the motor at low voltage submerged in a glass of water, for a few minutes. (Motor only, not the batteries!) You can also attach the motor to a slave,took with a piece of fuel tubing and run the slave low speed for a while. This helps seat the brushes and doesn't generate heat. Also, also, search for the "silver can tips and tricks" thread. It's like reading hieroglyphics, but the answers are there.