I run into a similar problem with an old Word document, where I only had a plain text version.
I wrote two small python scripts to help me with this: The first one takes all references and searches for them using Pubmed. The resulting file can be imported into a new Endnote library, which can subsequently be used with the second script to convert all numbered citations in a Word file back to Endnote placeholders aka curly brackets. Endnote will detect this and if used with the new library will fill on all the missing information to allow editing of the citation list.
The scripts can be found on github and are free to use and modify github.com/TischerT/endnote_convert