These were the first gear cogs in my '76. I didn't discover the problem until I went through the carbs and wiring in order to ride it the first time. 1st gear kept "skipping," and I then knew a transmission repair was in order. I have spare engines around, so I just swapped in some good parts, including a good 1st gear shift fork; you can see in the pic the fork was damaged, but it's a chicken-and-egg dilemma. Did the fork wear and cause the gears to jump, or did the jumping gears damage the fork? I also installed the taller ratio 5th gear while I had it apart.
You can see someone was in there before me with a die grinder and epically failed at "repairing" the damage. He also had the clutch washers in the wrong order, used way too much sealer for the cases, and forgot to put the spacer in the starter drive on the left side of the engine. It kept acting like the bendix spring was weak, but I was watching it and caught the fact that none of the gears on the right side were moving and the starter motor was spinning happily away. The last pic is the missing spacer and spring washer from my donor engine ready to install, but yeah, this was the spacer that was missing. If the "mechanic" who made this mess was there, I would have choked him.