Stepper motors almost never fail, so I'm curious what is wrong with yours? It may not be the steppers at all.
Regarding larger motors. Without changing anything else, moving to larger motors can often reduce performance. But it's hard to say without knowing all the specs of your machine.
Stepper motors rated torque is holding torque, when they are not spinning. As Rpm increases, torque decreases. With larger motors, the torque decreases faster. SO it's possible the smaller motors will have more torque at higher rpm than the larger ones.