Is it possible to change the speed of an AC motor(drill press) without losing too much torque?
You probably have a single-phase induction motor.  Variable speed is done all the time with 3-phase induction motors for industrial applications.  The device that controls the speed is called a Variable Frequency Drive or VFD.  Like it sounds, a VFD varies the frequency of the AC that drives the motor, and the motor changes speed accordingly.

Look at some industrial supply places to see if you can find a VFD with single-phase in and out.  Once you get the hang of what's there, buy one from ebay so that you don't get the 100x markup of a new one.

Also note that a motor has to be over-built to some extent to handle a VFD running slower than its native line frequency.  Those that aren't over-built will still work, but they may overheat more easily because the internal fan doesn't work so well at lower speeds.