Went through this same problem about two years ago. Couldn't find one anywhere so I called up Orion to try to get a replacement and they replied with the following -
"The sensor board is not something you can purchase separate. If the speed controller is old then you might wish considering purchasing a new esc. If you a hobby shop in your area take it there to see if they can do some testing to find out the specific issue."
Basically you're SOL. Pretty disappointing.