Union Jubilee Clutch.

[Refugee]

The easy method to get the speed pot to cover the full range would be to run the machine up to full speed and then stop it with the switch and measure the actual value from the stop point to where the wiper is. Buy a pot of the closest value and a range of resistors close to the value from maximum speed to maximum rotation of the pot. Then select a resistor that gives maximum speed as close to full rotation as you can.
The tolerance of the total value of the pot will most likely be 20% so a little bit of variation in the value will not damage the controller.