While it might be possible to re-create such a tiara, the major problem would be sourcing the large diamonds. If they haven't been re-cut, I suspect they have disappeared into a collector's private safe.
The thieves sound like low types. See this item from The Guardian newspaper https://www.theguardian.com/uk-news/2022/apr/11/ashley-cole-targets-gang-stole-375m-tiara-portland
Our fellow poster, The Royal Watcher, has an interesting article on the tiara on his site see https://royalwatcherblog.com/2018/11/23/portland-tiara/[/b]