Let's start a discussion on this subject.
If it's a worm gear loco, it can be converted, but having done two, it is not economically justifiable. Figure about $50 for a can motor from Frank Timko, plus an MTH upgrade kit. (I have posted a thread on doing this job on a #624)
If not a worm gear loco, but the old side plate design, that's a challenge. I can concerive of some possibilities:
(1) One could replace the armature with a spur gear, and try to fit a can motor with a worm on the end of a long shaft. Major effort.
(2) Can a tach tape be wrapped around the armature poles, and clear the field magnet and be read by a reader slipped between the field magnet poles??????? Theoretically, a full-wave bridge rectifier between a brush and the field magnet would convert the series-wound motor into a reversible motor.
(3)Is there some place to mount a disc on which the stripes could be drawn?????
(4) At last April York meeting of the DCS user group, MTH made mention of working on a device to convert old locos without using a tach tape, admittedly without precise speed control. I have heard nothing more on the subject.