A friend found this link to an excellent exploded parts diagram with parts list on Lionel support web pages:
Lower down in post with rewiring discussion is photo of motorized tower's bottom, so wiring can be seen to learn where there is slack to be had to remove tower top.
Have disassembled and adjusted "lash" of bevel gears on motor output gearshaft and both ends of drive shaft that runs tower-to-tower. Motor output gear lash adjustment shown before/after below. Bevel gear shafts each have two Allen setscrews 90 deg apart, using 1.5 mm Allen key. "After" photo shows gear moved out on shaft 1/16" or so for more engagement with driven gear. Same was done with bevel gears on both ends of long driveshaft that transfers motion to second tower gears.
With lash tightened, within perhaps 30-40 seconds of testing total first at 12vAC then increasing to 16v, motion became sluggish one direction and nearly nil (stalled to creeping) in the other. Disassembled again, felt for heat in can motor and electronic control/driver board. One component shown in photo (bottom center of board) too hot to touch.
Since motor is simple DC can motor, decided to toss the board as unreliable and rewire for DPDT center off momentary switch, eliminating the electronics. Bottom of bridge's motorized tower with 3 screws removed to take of cover plate, showing where there is slack in wires that go to top of tower:
Wires travel in black spaghetti tubing in clips along inside of one corner of tower, through hole into top housing. Can be carefully tugged from top to get some of the length where needed to disassemble things up there. Tower detaches from top structure with two screws on one side, and two tabs that disengage by rocking the top on the tower slightly after that. Top structure upside down looks like this at that point. Four screws hold the main portion of the floor of the top to the rim with railing, and four screws inside of those hold the motor housing to the main floor section:
Remove "floor" section first, then motor/control board housing from floor next. Then four screws that hold two halves of motor/board housing come out and it splits in half. Seam is visible running across bottom of the housing the long way, with four vent holes on one side, five on the other. Inside in two compartments are first, the motor and its reduction gearing and output bevel gear, and second, control board (see photo of failed board above in half of motor/board housing.)
See wiring photo, tower bottom: Two white wire leads from accessory power input terminals travel up the tower to the control board. After getting needed slack, those were snipped off close to the electronics board, and the two leads on the motor likewise snipped close to the board, leaving max length for all 4 wires. Then the white wires each were connected to a motor wire, so reversible DC power can be fed to the accessory input posts and travel by the white main power wires directly to the motor. This sacrifices the automatically illuminating aircraft warning lights in those posts on top of each tower.
A DPDT center off momentary switch will be used, with rectified DC from one of my accessory circuits yielding about 12 volts DC at the motor. It seems happy at that voltage. I tested operation on DC after all of this work. There is still some intermittent skipping at the motor output bevel gear, but it is fairly minor and there is not much left to adjust. The output shaft has a touch of room to "rock" in its bushing, which would require some sort of added sleeve between bushing and gear shaft to snug it up.
I've disassembled and reassembled this bridge enough, I think; screw threads in plastic will only tolerate it so many times before I make other problems for myself. Seems "good enough" and operates reliably now. Will be curious to learn from others who may have gone through this in some form.