I got an old Craftman garage motor. It has the torque I'm looking for. I got a LifeGear rowing machine on Craigslist for $50. I like this one because the seat and track is used for the leg exercise, moving the leg back and forth. The extra arm and pullies I use for the arm exercise, using her good arm to move the other arm. That way, both arms are getting exercise.
The base is built out of 3 in. PVC pipes. Since she's only 115 pounds, it's more than enough to hold her weight. And PVC are super easy to work with.
Using a Raspberry Pi and a Gertboard to control the motor, I can set how far back and forth to move the leg assembly.
My next step is to hook the Raspberry Pi to an iPod and let my mom control the speed and duration of the exercise.