Been there, done that...Circlips and their grooves are a must if you ever crack the tranny case open. I had mine rebuilt with the groove and a taller 5th gear at the same time by an independent wrench in San Jose Ca. for about $250 plus parts.
I heard a well placed source mention that there were some of the later model R100GSPD Classics that had the circlip re-installed from the factory.
"He had that rare weird electricity about him--that extremely wild and heavy presence that you only see in a person who has abandoned all hope of ever behaving"normally"."
"Woof, woof woof woof ...woof woof!!!!!!!!!"
Cricket the Dog-RIP