There are numerous places to find items to use, swap meets and flea markets often turn up oddities. Another great source is flea bay, just put in AMA pins etc. and you will find offerings of all shapes, sizes and years. Indian head or buffalo head coins are another interesting thing to use, just get the correct year for the bike.
I would have him check his shifter before you put hours and hours into making a knob to find out someone buggered it up and rethreaded it or wanted to use a knob they had that was a different thread and ran a tap over it. You will find out, and so will he that it is rare to find a motorcycle that has been around for 80 years that doesn't have many items that have been altered and often not for the best. I have a 37 and a 38 scout that I road race as well as a couple of 40's HD's and it is scary what you find.