As a man, I would want this in writing. Becuase if not it leads to…
I would pay good money to find out why this is so common. Oh, and why mothers do it too.
I think maybe you should just tell him the truth about why the piece didn’t mean that much to you. You can tell him it was a gift from an old boyfriend years ago (you don’t have to say “fiancé”), and that you would actually appreciate it more to receive something that reminds you of HIM than of a dead relationship from the past.
The problem right now is that he feels like he has to fix everything so it’s just the same as before (guys are “fixers”) and he feels like not getting exactly the same piece would be akin to an “incomplete” repair job. That’s why I think you should tell him that something that was more personal from him would not be a “lesser” repair job but an improvement.
I don’t think you have to worry too much about how he’d react to finding out it was a gift from an old boyfriend. Guys like to hear about how much better they are than the old guys. This gives you a great chance to pump up his ego by telling him how much better his taste is than the other guy, how much more you enjoy looking at it, etc. Trust me. It’ll work.
I have not seen where you’ve spoken ill of him here. Might just be easy enough to point him to this thread and say "read this.’
Light bulb goes on, boyfriend understands, all is well.
Well, I spoke to him again last night and he didn’t bring it up, which is good.
So either he’s going to let it drop (Good!) or he’s plotting how to find the replacement (Not so Good) or he’s hand-crafting the replacement himself (Weird, But Sort of Good!).
FWIW, when it happened I told him it was from my ex-fiancé and therefore the “giver” was not a source of sentimentality for me. I kind of think he was so horrified that he was hell bent on fixing it RIGHT FRIKIN’ NOW. I hope he’s chilled out a little bit. 
And of course I haven’t spokent ill of him. He’s a sweetheart. (Barf bags are on the left).