You can contact HiFiMan and buy new earpad lock-in rings and ask for slower postage for a cheaper total price or ask user Modulor on here (search his name with @ in front on the search bar up top) if he has spare rings....he made custom earpads so may have some lying around.
He is in USA and I'm guessing you are in Canada...but from China may be fastest.
Regarding vintage amps, you may lose on dynamics if the capacitors have dried out over time. My vintage stuff local place advised me against one of his amps due to lack of dynamics as there were very old caps inside.
You're taking about the lock ring inside the cup right? Not the one with the pad. How would I attach it once I get it? Superglue?