You could just run the motor off of a transformer or a variac.
The mains could be rectified with a 1:1 transformer and then a custom-designed inverter with a tunable frequency used to adjust the speed.
As was said though, it's not going to make it noticeably better. Do you know what the deviance of line frequency is? Not. Much.
Sorry, it's going to make it noticeably worse because the mains frequency is much more reliable than the voltage.
I can think of two ways to make a crystal controlled motor:
Use a stepper motor with the driver driven from a crystal oscillator.
Use a standard PM motor connected to a standard PWM controller, with a tachometer on the shaft, use a PLL and divide by n counter to synchronise the tachometer to the oscillator. I'll post a block diagram if you like.
I don't see any point in this because the mains frequency is normally very accurate. I suppose if this is going to be used in a developing country with a poor power supply or run off a generator then it might be a good idea, but if it's not, don't worry about it.