There are lots of other 304's around at this time, and since the Langford is not primarily serving in a warship role, it does not make sense to have any spare ZPM there. The Asgard power core is more than sufficient to make it a suitable threat against most opponents.

The real problem with bringing in systems like cloak is that its not understood quite well enough to be efficient enough on a ship the size of a 304. A cloak could probably use same sort of emitters as a shield, but that could be limited to lantean-class emitters. The Asgard, so far as we've seen, have not been very much into cloaking ships.

If we really wanted the ship to be nigh-indestructible, we'd have a merlin cloak on it, not a regular cloak. But as Forsythe said, that sort of thing would be boring.