I looked at the TR55 tracker and it looks like they are using Satellite dish movers. Linear actuators.
http://www.altestore.com/mmsolar/oth...tion_Guide.pdf
Those things are available on Ebay.. I use one on my DIY tracker.
http://ecorenovator.org/forum/projec...r-project.html
Edit:
Never mind.. I just found a pdf for your system. It looks like a special motor..
http://www.wattsun.com/pdf/Wattsun_T...tion_Guide.pdf
Have you tried to repair the motor? If the coils are not melted or open,
maybe it's just an End-limit diode or switch?