It was only shown that Magnus was a rank when Sentinel was told that he would "make a fine Magnus someday". So Sentinel is going to be the next Magnus when Ultra Magnus goes offline. (Hmm, Sentinel Magnus? Creepy, no?)
The Magnus Hammer was stolen by Shockwave after he put Ultra Magnus in stasis; it was reclaimed by Ratchet who said it was property of the current Magnus, not the Cybertron Elite Guard and he would keep it until he was repaired.