I was looking for a used composting toilet for a while just to see what they sell for verses a new one. Not very many available. After doing some research I think that if I buy one it will be a new Natures Head for $900. Couldn't you replace the motor? I'm assuming you mean the fan motor. I wouldn't think that it would cost a whole lot.
Watch out for the "natural" ones, they put two at the old CCC camp off sugar valley narrows road in bald eagle sf and its been plugged full since September of last year, guess too much use not enough time?
I have a Sun Mar Centrex 1000 and am wondering about your agitator? I didnt know any came with that option. My agitator is a drum handle that I rotate manually. I'd think you may be able to get a different model drum to use the toilet part. Good luck in your search.