So you need to turn up a short stub, with a 5/8 diameter and 5/8 thread at one end, a plain 3/4" section at the other with probably a reduced diameter part on the end threaded 5/8". The stub is retained onto the quadrant with a 5/8 nut on the 'far' side of the quadrant, the gear slips onto the 3/4 plain section, with a fat washer followed by a 5/8" nut.
You will also probably want to make up one with a double length 3/4 plain section for pairs of gears pinned together, though sometimes a sleeve is used with a keyway to fix two gears together