Rebuilding the back end can be time consuming, but can be done. I'm not sure what the Cat crankcase costs, but it ain't cheap. I'll call and get a price tomorrow, and let you know.
Here's the service manual...
http://www.catpumps.com/select/pdfs/CPSvc.pdf
Here's the parts...
http://www.catpumps.com/select/pdfs/5CP3120.pdf