I believe the pto switch part # is 725-3022 or 925-3022. Both same part...
Not able to find specs on the size of the nut...however if you log on to ebay and type in either of the part #'s above, You will find them for $10.99 each (free shipping) sold by EPR Distribution. Switch comes with the nut you need or replace with new.
Maybe someone who repairs old stereo equipment might have something that will fit as well... Good Luck!
