Stay with the servos! Steppers that would equal the servos on torque will not be easy to find and there no stepper that could match the speed. I have a SM1 that I updated the controls as my controller was bad my drives and motors were good. I would never put steppers on that size machine.
Depending on your budget and time you have seveal options if you think you may have a bad motor get one drive and test all three motor as they are the same motors. If one is bad replace it with a new AC servo drive and motor. oterh wise then buy the other two drives. I know a guy that used the Viper 200 drives.