You'll be luck to get a part for something that age.
I got out the loose concrete, wire brushed the reinforcing and built up the concrete with layers of cement mixed up with PVA trowelled on. Lasts a few years anyway with minimal cost. Helps if you can give the finished job a coat of bitumous paint.
Used a cup wire brush in angle grinder.
There are special kits for this sort of thing used formotorway bridges. No idea if small quantities are available. Google reinforced concrete repair kits. Epoxy resin and the like.