Old track-powered LEGO train motors have lots of plastic clips along the bottom, making replacing the motor and gears difficult. Newer battery-powered LEGO train motors are a great source of compatible motors, and their bottom plates are attached with Torx screws, making them easier to open.
To make opening and closing of old motors easier, you can print two of this model, cut off the clips, and glue each of these blocks inside the ends of the motor. Drill four holes in the old motor bottom plate (you can even temporarily clamp this block right inside the plate's lip as a drill guide!), and use the screws from a new motor to re-seal the old motor.
The author marked this model as their own original creation.