Short answer is YES it could be done. And you could indeed control motor direction by reversing polarity to the brushes. However the 3-pole armature would still produce big changes in back-EMF as it rotates, which would likely frustrate attempts at closed-loop speed control. TrainAmerica experimented a LOT with this, and the 3-pole armature was a deal-breaker.
Marklin locos are built a lot like postwar Lionel. In the late '90s Marklin offered an upgrade kit for their vintage AC motored, spur-geared locomotives. It actually replaced the wound field assembly with a new one containing a rare earth magnet. More importantly, the kit included a FIVE-POLE armature to complement the new field. (Google "Marklin Five-star high efficiency propulsion" for details.) I would love it if Lionel offered something like this to upgrade a postwar 2046, for example. But I've been waiting 20 yrs, and unless Neil Young makes it a personal project and gift to the faithful, it probably won't happen.
Another approach is to replace the field AND armature with a "pancake"-style can motor. See this thread for an example: https://ogrforum.ogaugerr.com/...-gear-2036-127-pitch
There are also a couple of threads on the forum about replacing Berkshire or Hudson-style longitudinal Pullmor motors with a cylindrical can motor. Frank Timko offers a service like this. But the can motor has to be pretty small to match the form factor of the Pullmor's offset field winding. And IMO a tiny can motor isn't well-suited to the tall gearing of a postwar loco. Good question, worth discussing!