There should be a series of 22mm bungs in the water jacket on the left side of the head. If your engine has a spare, simply remove the plug and install a fan switch from a VW or Porsche and wire it in parallel with the fan switch on your receiver drier. This is the hottest part of the cooling system, so you probably want to go with a 92C switch.
https://www.pelicanparts.com/More_Info/823959481FINT.htm?pn=823-959-481-F-INT&bt=Y&fs=0&SVSVSI=2100