@Jon Elliott
I believe the old style Hunter was built between 1978-1984. It contains the 1GM model Yanmar which is about a 7.6 hp engine while the 1GM10 is a 9 hp but the manual on line for Yanmar will work for both.
The dealer for your area is Mack Boring and Parts Co.
I just fixed a 1 gm on a 1981 Hunter.
Do you have the original water separator as it will be solid metal? If so I suggest replacing it with the clear bowl on the bottom so you can see. In addition you will have to I suspect replace the hand pump as it needs to being 41 years old. Here are some numbers for you:
Item Model number
Fuel/water separator 370-4120
Fuel filter element (GM/HM engines) 370-104500-55710
Filter assembly 370-119305-35170
O ring (fuel filter) 370-24341-000440
Hand pump 370-105582-52010 (order gasket)
Impeller kit (ask if gasket is included) 370-128176-42080 (if not order separately)
The hand pump does not come with the gasket so order it. Check if impellor kit contains the gasket. Replace lines from gas tank and thru the system. Do not disconnect line that attaches at the injector. Remember this is metric, so have both standard and metric tools.
do not take the fuel shut off from tank as that will be a bear to do so.
Contact info for your area dealer is:
Mack Boring
245 Belmont Drive
Somerset, NJ
908-964-0700
Send me a private email stating H 27, 1978 model/1GM engine with you phone number and I will discuss what I did.
Crazy dave