The best thing to do would be to ask who wound it. If that fails, ask Ricky Stator how they rate their stators, in terms of power rating, and see if you know enough about the new stator to calculate it.
I wouldn't recommend trying to load your stator to find the rating. The armature wire that they are wound with has a very thin insulation on it to keep the wires as tight to each other as possible. If you try and pull too much current through the stator, the wire could heat, melting the insulation and shorting the winding at that spot to an adjacent winding.
Additionally, loading the stator won't give you a safe continuous wattage estimate.