There are rebuild kits available but I think they only contain seals, bearins and gaskets. Your gonna have a difficult time finding gears and other parts I think. Give Boyce Equipment & Parts Co. a call at 1-801-393-0007,
www.boyceeequipment.com as they are the ones that priced me the rebuild kit a few months ago.
You might also call 4X Heaven at 1-800-800-1679,
www.4xheaven.com
Lastly, e-mail member Gregsplace as seems like he might have a Rockwell sitting around for sale.
If it were me and I was faced with your problem, I would do my best to switch to the NP205. I'm keeping my Rockwell as I switch to a 4 speed, but just because it is working fine.