How can I fix my Kew Hobby 1500 Pressure Washer?

On taking apart my Kew Hobby 1500 Pressure Washer to investigate a bad noise, I discovered that the motor cooling fan had come off the spindle. It is secured by a washer and nut, which I cannot find anywhere and am therefore trying to source a repacement. The threads appear to be left handed and course thread. Kew sites do not list these parts. Doesnt need to be a Kew part any left handed nut of the right size will do. Any suggestions?