I would check the armature by changing it to a good used one if I suspected the armature. If it goes OK in one direction I would be tempted to say it is good. How much forward/back movement does armature have, can you see the commutator distance from the tubes?
if it looks OK I would then be looking at config of the wiring to the reverse unit compared to either a diagram or a known good worker.
What number is the engine? You say one brush was jamming, did you take both brushes out, clean tubes, clean brushes and make sure springs are OK? If not then it would be a good idea to do so. It could be that the brush is not making contact properly in reverse, could be it makes contact just in forward and is just light in reverse and is causing the arcing.
