I have absolutely no experience or knowledge of any of the Aero models, but in my experience sluggish starting is much more likely to be caused by a poor earth strap connection than it is by a duff starter motor.
Please forgive me if I'm teaching granny to suck eggs, but have you checked the integrity of the earth connection from the battery? They can be visually fine but still offer high resistance. IT's got to be worth changing this before you go the the complexity and expense of swapping the starter motor, I would have thought...
