Replacing or remounting that tag can be a federal issue and I would be very careful going down that path. I would not and have, in the past chosen not to and scrap cars where they were missing. Someone may make them but its a bit hush hush and you could anticipate allot of questions, verification and hoops to jump through if they do supply you if the company above board. There are past threads here and on other sites about experiences and warnings about touching VINs
You might want to consider just using an overlay decal to freshen it up. I found that cutting the center out, so that the clear layer does not extend over the stamped numbers and letters creating pockets and air gap. Though not perfect IMHO does improve the look
Decals were available through most of the suppliers like NPD and Virginia Mustang