The casings might be a different size. I know the last servo I stripped down was well packed in. The motor was in a sleeved section of the casing.
Also the circuitry might not be able to provide the current a faster/stronger motor requires.
And with servos, same as cars, the speed and torque is related to motor winding, gear ratio and current draw/voltage.
It's probably worth trying for a bit of an experiment, but I'd not try it with a good, functioning servo. I'd buy a cheap/knackered one and see how you go.
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