As far as I know there aren’t any tools to do that, Lemmy doesn’t automatically fetch older / deleted posts from federated communities like that, the reason is the way that federation works. When a user subscribes to a community on another server, that server will then start notifying their server when there’s new posts, comments, etc to fetch, but Lemmy doesn’t fetch older content, so only new stuff from that community will show up. I believe it’s possible to fetch old posts and comments using the search function though, you can read a bit more about it here: Lemmy docs