I think that it is the roller(#5 on the fiche) that allows the shift fork to follow the groove in the shift drum. It's hard to say how it fell off the pin it runs on, but I do know that when I was putting my transmission back together after tightening the grub screw one of them fell off and got hidden under a gear. Almost didn't see it, and it took about a half hour to get the damn thing out. I suspect it fell off when someone was putting your transmission together and they didn't notice it.
Things will work, just not nice and smooth without that roller. Eventually, the bad shifting and the part banging around in there will make things stop working. Have you checked the gear teeth for damage?
As far as replacing it, shouldn't be too hard. Just get a good manual and take your time and a lot of photos as you take things apart. You should be able to get the drum out enough to get the roller in without having to pull any of the shafts with the gears. I did the grub screw on mine in one day from start to finish. and that included putting that roller back in after it fell out.
I would suggest making a support of some kind to hold the bottom half while you work on it. It will make things easier for reassembly. Make it before you take anything apart.
