I dont know if this information is of any use but it comes from a genuine 1971 lister generator brochure.
              Fuel consumption LB/BHP/HR =0.5
This is for the 6.1 and 8.1 engines it comes out of a genuine lister brochure can we believe these figures

Mick

Have you considered a cruise control unit off an automobile it has a linear actuator a computer to operate it and picks up its signals from the speedo or rev counter electronic pulses a magnet or 2 could be glued onto the alternator pulley and a hall effect switch used to provide pulses for the triggerwire to the computer these units are very tough they are made to live in an engine compartment