You should be able to size a replacement motor by matching the stator size and the kv of the original. If the original kv is not known, it can be calculated. Stator sizes are usually read in mm and called out. For instance a 2206 motor is 22mm in diameter by 6mm in height. So in a pinch, in theory, you should be able to get away with using motors from different manufacturers as long as you match the size of the stator and kv of the other motors on the quad.
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