. . . by "the armature windings look crispy."  Unless that black area is a burnt spot, I don't see "crispy."  Armature windings are often a different shade of color than the field windings.  Given your description, the problem sounds to me to be mechanical, not electrical.  I say that because I recently acquired a PN-4 that wouldn't turn at all and just hummed.  If the windings in your PN-4's motor were discolored from overheating, I would be more concerned that it's an electrical problem.

I would tackle this problem in this way.  Run the PN until it's slowing down.  Then check to see if the brush roll is hard to turn.  If so, it needs disassembly to remove all the hair that's wound around the shafts (yeah, it will be there, and yuck), and re-lubing.  Even if it's not the brush roll, you'll want to do that.  Put a drop of oil on the shafts, and also saturate the felt pads that feed oil to the bearings.

If it's not the brush roll, is the motor hard to turn?  If so, it can be taken apart and lubed.  It has sleeve bearings on both ends because the toothed belt doesn't exert side thrust on the front bearing.  This was the case with the motor in my PN-4.  I lubed the motor and now the unit runs perfectly.  Like the brush roll bearings, the motor bearings have felt pads to hold oil.  You can only get to them when the motor is apart.  If it's not a mechanical issue, the motor probably has a short that presents when it's warm.  That will be evident from excessive sparking at the brushes.  In that case it's not worth having it rewound when there are many PN-4s on auction sites from which you can pull a motor.

As an aside, the rear bearing in your SJ's suction motor is a sleeve bearing that, if properly lubricated, will outlast the rest of the motor.  If you're mechanically inclined, the front bearing is a 608 (I think) and isn't difficult to replace.  If it's like the front bearing in late model L's, you'll have to drill out some rivets and replace them with screws.  Maybe intimidating for first-timers, but once you've done one, it's a "meh."

I hope this helps.

Joel