An all aluminum head can gave have heli coil installed in shft hole then drill out whatever hole size is needed-The drill has to be a perfect 90 degrees to head plane.
Any machine shop can do this-
I will go measure my brent wheel head banding wheel made from old head for shaft size diameters-back with that soon-need to mix some glazes today anyway.
I measured my old Brent 14 inch (from the 70s) wheel head converted to a Banding wheel.
The old Brent shaft is 5/8th exactly what you are looking for.I have not measures a newer Brent shaft-they may be bigger-I recall that that may be the case.
How about calling Amaco/Brent and talking to the Brent technical about a 5/8 wheel head
The brents are held in with a roll pin ponded in and never removed except drilling them out as they are at an angle
You could drill a new hole at a 90 in the head and get a roll pin from brent as well for the new head you order?
Options many options.