Knife safety repair

I'm looking to repair the safety slide on this knife. There use to be a hemisphere type of thing on it similar to the button. Years ago it randomly fell off and I didn't notice so I lost it. Any ideas on a good repair? The knife holds sentimental value to me as it was my grandfather's and I believe he got it in West Germany when he was stationed there.

If anyone knows of any better communities to post to for help I'd love the recommendation!