While Bowser motors tend to be sturdy and long-lasting, if they fail in your single truck Birney you're up the creek as far as getting factory replacements, since Bowser doesn't make 'em any more...
Here is the Razorback Traction Co's Bowser Birney with two blown power trucks. The DC65 (center) threw several commutator sections, while the open frame motor has a burned coil. We'll be using parts from both trucks, as well as a 1960s Japanese slot car motor to make one complete working truck...
Initial disassembly, showing the underside and the worm gear on the open frame motor.
Most model train wheel/gear pullers are modified automotive battery terminal pullers, used when the battery cable on your car is corroded tightly to the battery terminal. If you don't have a gear puller handy, a stock terminal puller will do the job (along with an appropriately sized piece of bar stock or a screw):
I experimented with a couple of different concepts to get the 1/8" bore worm gear onto the metric shaft:
Eventually, I wound up epoxying a commutator from one of the dismantled slot car motors into the worm gear.
Test fit using one of the red motors.
After removing the open chassis motor and bearing, the floor was drilled out to accommodate the motor:
After some trial and error (mostly error), the final motor selection was mounted in place. The top plate (sans top bearing) and assembly screws of the open frame motor were used to hold the slotcar motor in place:
A bottom view with the worm gear installed. The ends of the mounting screws still need to be ground down so the gearbox will snap into place.
The wiring harness and gearbox from the DC 65 chassis, along with its rectifier bridge and lights, installed and ready for connection of the slotcar motor leads:
And here's the completed Birney chugging along my test track! 
After I took this snap, I wound up swapping in the gearbox from the open motor car (brass wheels, image rear) as it ran more smoothly than the iron wheeled truck...
And that's that! The Birney now scoots about as enthusiastically (if not as quickly) as a slot car... 
Mitch
