That ‘hole’ is a drilled passage that runs the length of the cylinder head and is cross drilled into the exhaust ports, that allowed fresh air to be pumped into the exhaust ports to help to clean up the exhaust emissions.
Should take a standard size cup plug, maybe 3/8 or 1/2.
The air in opening is to the rear side of the #6 intake port, it also should be capped or plugged,
