Existe uma maneira de fazer um link magnético para baixar arquivos específicos de um torrent?
Não. Links magnéticos não sabem nada sobre arquivos no torrent.
A magnet link is essentially a hyperlink containing the hash code for that torrent, which your torrent client can immediately use to start finding people sharing those files. Magnet links don't require a tracker (since it uses DHT, which you can read more about here), nor does it require you to download a separate file before starting the download, which is convenient.
Fonte O que são links magnéticos e como usá-los para baixar torrents?