Head Gasket dimensions

Sorry if this is off topic a bit (quite a lot really), but I wonder if a Commando owner out there might be able to help me by measuring an old head gasket?. I have a veteran bike (not a Norton) - single cylinder, with a raised spigot on the barrel around the bore. I need a new head gasket, and the land is very narrow. Solid copper doesn't last. I see the outer diameter of the raised spigot is 73mm - which is identical to the bore of a 750 Commando. So I'm thinking about butchering a Commando flame ring type, or anyway one with a raised lip around the edge like copper-asbestos (or whatever they use now) to provide a bit of 'squash' where it counts.

I don't need to consider oil drain holes or anything like that.

I just wonder if the hole is actually 73 (less nothing and maybe plus 1.0), and also circular? I might as well send my cash to one of our Norton dealers as try elsewhere. (My Nortons are 500)