Smelly-I built this exact prop 3 years ago. It doesn't have the movement that Dr Morbius's version has, but it is a cool prop. If you decide to build it, I would be willing to answer any questions that may arise.
The wiper motor idea is great, but not the effect I was hoping for. I don't want thrashing, but rather jerking motion of the legs. I can't use compressed air, so I think some kind of selenoid. Hmmm, princess auto has an electric nail gun with that kind of mechanism build in it that runs on 120v. looks like they get to see my face yet again. Plus I need some power supplies for my wiper motors.