Use the key from the public tracker (log out and grab it from the torrent page) to finish then change it back to yours (you don't get credit for seeding on public keys, but it doesn't track your ratio at all).
You can still seed without having the whole file though, it just won't list you as a seeder. You'll still get credit for uploading though.
With a upload speed that terrible, you might wanna consider looking into a seedbox though...
