The 300 RPM motor is the same size as the 600 RPM motor. But you need a larger pulley wheel. Whether there is room for that I can't tell you, but there probably is. VPI will sell you one.
To use only the SDS to get the right platter speed without changing the pulley will simply return the motor speed to 600 RPM but it probably will not run as efficiently and quietly as your present motor.
To use only the SDS to get the right platter speed without changing the pulley will simply return the motor speed to 600 RPM but it probably will not run as efficiently and quietly as your present motor.