I can only recommend a three phase motor (240 volt) to replace your single phase, and an OMRON inverter to drive it. This gives very precise control of speed, because it works on frequency, and is programmable. My motor is about twice the size of the old one - about 1 3/4 HP instead of 1 HP to get over the loss of torque at low speed. I got mine second hand from my brother in law who is a factory electrician and most of these places have stuff that they don't use anymore. The Omron had apparently been in for repair, but was never used again.
My only problem is that it is not fully compatible with Mach3 - I have G3 G4 and G5 working (on, off, reverse) but the actual speed is controlled from the Omron panel which is next to the computer anyway. This is the fault of the Omron,(which is an old model) not Mach3. It is possible to control speed on it directly, I just haven't got round to fixing it up yet!