I bought the MOT-SD36600 36v 600W motor, expecting it to be a good match to the stock controller, but I've encountered an issue -- it's noisy as hell. It came with a eleven tooth sprocket for a #25 chain, and runs nice and quiet without the chain attached. Put the chain on, it sounds like a buzzsaw. Something isn't meshing properly.
(photo - new on left, original on right)