How do I replace the motor on a LitterMaid cat box?

This is the LM900 model, the larger size model -- the new and improved tan model with the under-powered motors that die young. Purchased in 2008.
Also, I want a better quality and heavier duty motor (more like what is in the larger blue LitterMaids, model year 2000). What motor should I use and where can I purchase them?
(Please don't be confused; this is Not the more expensive model with the air-cleaner that doesNot scoop-box-contents-into-the-receptacle.)