I added a 150 ohm resistor to the ground-side of the speed control potentiometer. I had done some measurements and that was the right resistance to let the lathe run at its lowest speed (48 RPM on the low pulley) when the speed control is all the way down. It used to stop the motor.
When I needed to run the lathe really slow, it was tricky to find the spot with the speed knob.
I may someday want to be able to turn the motor off with the speed control again. If that is the case, I’ll get a switched potentiometer to replace the original one. Then I can turn it down all the way to get low speed, and just click the knob to switch it off.