Both holes in this yoke should be welded closed and redrilled to the diameter of the pins desired. The front hole will carry the pin that goes through a clevis on the rod that connects it to the treadle, and the rear hole will carry the pin that acts as the pivot in the frame.
The same is done for the holes in the treadle. Remember that the treadle and the pin connecting the ends of the treadle should move as one piece, so plan for a pretty snug fit in the treadle ... 0.001" ... large lock washers are used between the nuts and the treadle itself. We now know the diameter of the holes in the frame, and the diameter of the holes in the treadle and in the yoke ... and we can make the pins. Good old CRS (cold rolled steel ... 1018) is fine for these pins.
Steps in the process are something like this:
1) Mount the stock, and bore a hole deep enough into each pin that it reaches the center, but is not so large that you can't thread it for a grease fitting.
2) Cut the major diameter of the pin.
3) Cut the steps to fit the treadle arms on the big pin and the step for the yoke on the small pin.
4) Thread the ends of the pins and make sure that nuts actually fit the threads.
5) Drill a vertical hole from the outside of each pin into the center cavity so grease can get from the center of the pin into the core around the pin, and finally
6) Thread the center holes for the grease fittings. (Each of these pins can also be made in several pieces if that turns out to be necessary ... so don't worry about the way you have to do it due to tooling limitations. Just make sure that the fits are snug where they need to be and have good float for lubrication elsewhere.)
At this point this is all probably starting to sound as if it's being over done ... believe me it's not. The change in the very nature of the hammer is dramatic and worth all of the time and hassle!