It may seem like a mismatch with a 12V supply, but you'd likely be best looking for a motor that matches the 2.8v 1.68a spec of your current motor, or at least is in that ballpark. The stepper motor driver takes care of the "mismatch" between your 12V power supply and the lower voltage motor. Places that sell motors for 3D printers should have something that would work.

Here's a good summary of stepper motors and the 3D printer application - http://reprap.org/wiki/Stepper_motor, including an explanation on why you want the lower voltage motor.