Omega has service crowns for this watch. The original may have been a shorter Naiad crown, which is likely harder to find.
Cousins UK lists two crowns, 42028ST in stock GBP19.96, and 42062ST as restricted. The former looks like the older, flatter crown, but your watchmaker should be the one to say whether it's suitable. I suspect the latter crown is the screw-down crown, which it appears is not suitable for your watch, hard to tell.
Tom
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