Ordered a new door frame morning of March 25. Part arrived the next morning and by that afternoon, the replacement was complete. Only four screws held the frame.
The microwave is the upper unit of combination microwave & conventional oven. Disconnect power to the unit. Slide unit out of built-in cabinet to expose the top shield of the microwave. Remove 5 screws holding top cover in place. Remove cover, which exposes the belt reel and drive motor. Install the new belt by threading it on the drive motor pulley, then walk the new belt on the stirrer pulley. Replace the top cover and reinstall the 5 screws. Reinstall the combination unit in the wall cabinet. Reconnect electrical power. Task Complete. The belt just seem to die of old age about every seven year. Without the wave disperser operating, the microwave oven does not give even heating.