I know it’s not an ideal solution, but it may well be the best you will get for now:
If you view your own shop, then the search feature at the top will get you the result you want.
For example, I searched for “40542” on yours, and item “285PC WALL PLUGS SET ASSORTED MIXED RAW DRILL PLUGS ANCHORS AND WITH SCREWS KIT” was listed.
You can then click to view, then click to edit the item.
Or now you know the item’s full title, (copy/paste) search within the “Manage active listings” section. (I would use this method. My browser – Vivaldi – gives an option to copy a link’s text, so I would have the shop search page open in a new tab, and copy and paste between.)
Again, I know it’s not an ideal solution, and it certainly doesn’t make up for the removal of this feature, but it may help speed things up for you.
Perhaps there is a tool out there that will automatically put the MPN into the “Custom label (SKU)” field? Then you can search that from the “Manage active listings” section.