Having years of experience in jewelry pricing, I am confident to say that value loss happens when you replace missing stone of your filigree engagement ring. But if you are able to replace with stone which matches in 4′cs (Clarity, cut, color and caret) the value will be restored or value loss will be negligible. You have to look for jeweler experienced in restoration and conservation work, not jewelry maker or seller, there are a lot of difference between them.

I think some kind of repair work is required to the filigree engagement ring, because the stone is lost and new stone needs to fit exactly. But the originality should be maintained. You may want an appraisal though you have no intention to sell it. You also require an appraisal if you have insured it and want to file a claim to your insurance company.

You can also get guidance from a good jewelry appraiser.