If you find the torrent files or the magnet links you used, and you add them to your torrent client again, it should be able to pick up your existing files and seed them (if you haven’t renamed or moved the files). Before you do that, make a backup of the files at least for the first few torrents, in case the client starts overwriting your existing files. In that case you may need to do something, like to start a “recheck” operation. If it hasn’t done that for the first few, it’s safe to assume it won’t either for the remaining.