I just replaced the back gear on the spindle cone pulley on my 9" Monarch lathe from (probably) the 1920s; I machined the old gear off, to a diameter that would allow sufficient wall thickness to lightly press on a new gear; I made the new gear of cast iron because I wanted it to run quietly, and avoid steel for that reason; I cut the teeth on the new gear, pressed it on, then drilled and tapped three holes parallel to the centerline on the interface of the old and new gear diameters, 1/4-20 tpi. and then screwed in pieces of bolts with Loctite and cut them off flush with the gear. Allen head setscrews could also be used to key the old and new parts together in the same manner. The same approach could be used for the backshaft gear in the project described above, but the teeth could be cut after the blank has been pressed on and keyed; I could not do that, because the cone pulley flange is larger than the gear diameter.