Is siporex some kind of gas/aerated concrete?
It was very common for basements where a load-bearing basement wall, for example, in concrete blocks on the inside was covered with aerated concrete with an air gap of a few cm in between.
That's how it is in my basement, which is original.
Everything depends on what you intend to use the space for. If it's not living space, I would definitely just plaster the existing walls.