Concrete is a peculiar material to work with, and repairing it can be quite troublesome.
The main thing is that it is structurally sound, even after 80years that is very good news.
In my experienced opinion the best method would be...
Mix up a stronger batch of plaster(more cement or less sand), typically the ratio is 5-1 sand to cement. But many times on the bad of cement you will see guidelines. This ratio is for indoor bricks. Because your wall is already rough, jagged it provides very good grip for the layer you are going to apply. Now remember how I said concrete is peculiar.. naturally you would want to seal the old concrete before putting the new one.. this is not a good idea because concrete needs to breathe and bonds better with other concrete applied onto it. However, sealing the TOP layer will be the crucial and most beneficial step!
You are concerned about the metal parts inside the concrete... no need. Once you apply the plaster and seal the top layer, either by oil based paints or some sort of concrete sealer, the oxidization levels inside the concrete will be minimalistic.. and for the metal to completely degrade will take a few hundred years..
You do not need to particualry prepare the old concrete.. it would not hurt by hosing it down with water a bit before plastering it. Will help with bonding. You can leave any loose bits in side- just try to fill up any voids in the concrete with plaster.
As long as the pillar in that photo is deemed structurally safe.. plastering it will make it look like new
A nice typical flat finish after plastering...
There are other ways of doing this but this is only how I would do it.