Ninjas and monks do not have anywhere close to everything in common. The monk speed, AC and damage by level increases would fit Ninjas. They both use Ki, I mean focus their internal energy to do stuff. That's where the similarities end, though. Ninjas are focused on hiding, vanishing, and assassinating. They wouldn't have things like the ability to do force damage, step of the wind, deflect energy or body and mind. They would have other abilities in their place. I could see a ninja with something like step of the wind, but that only/also affected their enemies. A ninja might force a victim off of a high wall to fall to his death in that way.