The speed sensor controls the speedo, and both odometers. Since the only one not working is the odometer, that is what you need. I have a complete cluster for sale. You can easily disassemble the odometer, and change the mileage on it to match, or get close yours. You could even swap just the motor and any gears to your original one.
It may have an internal problem, but I can not troubleshoot that far over the internet. Post your location, to help us know where you are in case you need something shipped to you.