You can carefully cut your own with a baseball and a drill press the old school way. Modify the baseball by drilling and inserting a piece of 1/4 rod so it will go into a chuck. You need to be well centered on the press to get it accurate enough that it does not wobble. Then use some cloverleaf grinding compound, Grade C will cut quick enough without gouging. Use the headgasket for the cylinder bore to trace the bore to the head with a sharpie. Chuck the ball in the press and center the head, smear on a generous amount of cloverleaf. You are only using the press for accuracy, let the compound to the work dont force it. If it sounds like the compound is not cutting anymore then wipe out the expired grease and add a bit more. Cut until you reach the drawn bore marker line and double check with the gasket. Continue until you have a match.
The compression humps will take a bit of the cutting, but those are in the way of good flow to the spark plug, which is also shrouded quite a bit and could take some rework as well while you are fixing the squish.