Although the question has indeed been asked before, there has yet to be a satisfactory answer to this. I have an SX3, and here is what I have done.
I am just finishing up a conversion of my stock SX3 motor with an AMC servo driver I got from Ebay. I am using an AMC B30A40AC drive, running on 220VAC, and have the drive interfaced to a Gecko G540 for on/off/speed control from Mach3; as well as a new front panel with a pot instead of the up/down switches on the SX3. As the motor is only 3600 RPM, this is not quite what you have in mind; but it is a known working solution. Options are to find a similar BLDC motor with a higher speed rating. One possibility is the Keling motor (Page Title - motor only - the listed controller has been deemed unsuitable) which is good to 6000 RPM. I may try one of these someday with my AMC drive if I ever find myself speed limited.