This part is clearly shown in the engine manual. It is described as ‘Cylinder head washer for tubular stud’. The part number is 27-795.
Have a look at pages 72 for the diagram, and page 73 for the description and part number. My manual is a different version to the one you are viewing.
I believe Stationary Engine Parts, Sleeman and Hawken, Marine Power Services or Tony Redshaws should have the part. Alternatively, Dobsons Gaskets can help, but they’ll need a sample/template.
It takes a dedicated person to find this stuff, but of course, once you’ve stripped and rebuilt one of these fine pieces of engineering, you know what to look for.