Bottom line, if the motor's brushes can be isolated from rail pickup and/or the frame, you are good to go. If you can ensure the motor only gets electricity from a decoder output, and nowhere else, even inadvertently or intermittently, you can add that decoder and get a DCC-capable locomotive.
If the motor is cradled or mounted in metal, you'll have to free it, possibly by milling the frame, and then suspending it in a plastic construction. Some use goop, a big dollop of it, and squish the motor onto the dollop so that its shaft aligns closely with any drive-shafts to which it must couple. Assuming the drive shafts meet with at least one plastic gear, you should be okay. If one of the drive-shafts is plastic that would constitute an isolating barrier. Whatever the case, that metal shaft in the motor can't come into contact with metal that runs in metal bushings cradled inside a metal block on the frame. You get the idea.