you could try using cross-seed to search your library. i recently set it up and it's been matching with torrents that i no longer have torrent files for.

I'm pretty sure this will be a lot of work. Even assuming you didn't rename any files and all the torrents are in a single folder, you still need to re-search for every torrent file and add them to a torrent client that should automatically find the files in the target location and check them to see that it's already downloaded. I'm not sure if all torrent clients do this so I would advise testing it first - when re-adding a torrent with existing files I usually add it paused and then force re-check to be safe.

In theory it should be possible to make a program that does this - automatically indexes and crawls torrent sites and DHT, fetches torrent meta info, then scans your HD for matching files to seed. This feature is a proposed one for the DHT crawler "bit magnet" so I'm hopeful that one day there will be a program that can do that.

If you were using the sonarr/radarr and have them recovered then you can check them, they should show you what torrent they pulled

This is probably wrong but maybe it can be done by using jjackett to search for the torrents and using the file's hash to verify

That's a tough one, if these are public torrents you may be stuck having to search for the torrents one-by-one and hope you can find their associated .torrent files to re-seed the torrents.

You do need a .torrent file, or bare minimum a magnet link to an active/non-dead swarm, to be able to re-seed torrents with existing data. Magnet links without any current peers on them are a bit useless for this task unfortunately.

If these torrents are from private trackers you can check the private tracker website(s), many of them have a feature to allow you to re-download all the .torrent files you previously downloaded from the tracker.

if you make a new torrent out of the downloaded one’s directory, the hash should match up and magnet should automatically be the same torrent you downloaded.
although your os probably threw some hidden files in there to fuck that up.
the main filename is usually unique and searchable though… and you could start the new download, let it create folders and temp files, then stop it and copy the completed files over, then manually recheck the torrent….
you’d need a new download directory of course….
(i’ve done this one)
futurewise you can have some programs automatically save a .torrent file for new downloads (qbittorrent, at least).