I can't imagine that there would be any significant capacitance from the bottom plate so your main focus should be getting the magnets right. Removing one coil and it's iron core will probably completely change the sound since the magnetic aperture will become much narrower. There's really no knowing what will happen until you try. I'd start by pulling the screws from the inactive coil and see if that changes the sound of the active coil. Since the magnets are ceramic you can change their aspect ratio as needed to accomplish your goal but you shouldn't have any trouble fitting the remaining armature into a P90 cover once the deleted coil is gone. The magnet(s) are generally glued in place so sawing off the extra width of baseplate shouldn't pose a problem. Start with an undrilled p90 cover in case you need to locate the poles off center.