Advice on this 2900-1 cal.267

Hey guy,
I need some advice from someone much more knowledgeable than myself. I purchased an Omega 2900-1 off of ebay. After it arrived, I had it checked over by my local watchmaker to make sure everything was what it was said to be. After further examination, when it was time to put the backing onto the case, we realized it wouldn't snap on. It had been glued. There was missing material on the case back. My question: will I need to find another case from a 2900-1 to be able to fix this? Or can I find another case back from a similar sized case? Do I have any other options?

I'd return it. Unless the issue with caseback was mentioned in the listing, I would say it's not as described. Could you link the listing?