Erm... both photos portray the same building, just from different POVs! The one from WW2 depicts the place's "rear", if we can call it such, while the other one is clearly the façade you can see from via dei Fori Imperiali.

Apart from the ongoing scrubbing and replacement of the old, make-shift enclosures with new ones, the building's changed very little in the last 80/couple hundred years - the only interventions of some relevance being Giuseppe Valadier's consolidation of the monument's outer ring (or what was left of it) in the early 19th century and the excavation of its underground in the 1930s.