The simplest would likely be a 10A 240V variac connected to a 10A bridge rectifier. The motor may work directly off the rectified DC without filtering, you'd have to try it to see.
You'd have to limit the rotation of the variac so that it wouldn't exceed the 180V rating of the motor.
You may be able to find a used variac on Ebay.
A possible alternate would be a 10A SCR/TRIAC light dimmer circuit connected to the bridge. You'd have to try that to see how well it worked. Again you'd need to limit the rotation to not exceed the motor voltage rating.