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.
Not sure if this is a requirement for you but if you are used to using your own VPS then you might be used to have root / SSH access. When browsing seedbox services you may want to double-check if their plan(s) offer that.