How can I get iTunes to redownload a deleted Podcast item?
Press the three dots right next to the name when you select the Podcast row. Select (-) Delete. That will delete the row.

Then on the main Podcast, right-click and select "Refresh podcast". The row will re-appear with a little cloud next to it.

If the publisher has deleted the Podcast from their library, I guess you cannot recover it.

Hope that helps.