Should be rather simple actually. You’d need a slightly modified version of Lemmy which simply ignores federated deletions.
Beyond that, you’d have to Index the fediverse somehow. This could probably be achieved by subscribing to all relevant communities. There’s ready made bots for that I believe for the purposes of populating small instances with content.
The legal side of that is questionable at best though, just like removedit. Slightly less I’d say since re-hosting content here is generally allowed which was certainly not the case with Reddit.