Look at http://www.kelinginc.net at their Nema 42 stepper motor. The shaft is smaller than the spec sheet says (the spec sheet size would have matched your pulleys) but only needs a new Taperlock bush to mount the pulley. I know it works for the Y axis. I don't recall whether the X and Z use the same type pulleys. The bolt holes match up but the new motor doesn't have threaded holes like the Sigma/Superior so you may have to use nuts and bolts.
A friend was having issues with table jerkiness. The Y and Z were both Sigma and the X was Superior. He swapped in a kelinginc motor on the Y and resolved his issues. The Z is still a little jerky but he is living with it. The Superior motor is smooth. I tried his old Sigma motor on a unipolar HobbyCNC board with various step settings and it was jerky on it also. It would only smooth out if the current setting was set so low it stalled too easily. He has Gecko 201s on his mill.