I think this script/button will do what you want.
For all selected files, if the file is an image and has Focal Length, but no Focal Length 35mm, it will write Focal Length 35mm as Focal Length x 2.
Fix 35mm.dcf (2.01 KB)
You can drag & drop the .dcf file directly on to your toolbar while in Customize mode. No need to open or edit it. See Button .dcf or .dob files near the bottom of How to add buttons from this forum to your toolbars for more detailed instructions.
Please try it first on some backups of your images in case anything goes wrong. (In fact, it's a good idea to keep pristine backups regardless, since metadata editing could always ruin a file if something does go badly wrong, and it's always good to be able to go back to the original if needed.)
Purely for reference, and to help anyone looking for similar techniques, here's what the script code looks like:
