Actually, you can get there from here.
Try this. Build a model in V5.3 and create its .CII. Take the same model to V7.0 and do the same. Compare the structure of those two .CII files. You will see what you have to remove from the V7.0 .CII to produce a V5.3 .CII.
CADWorx creates the ".C#2" file. I don't think you can tell CADWorx 2014 to create a V5.3 ".C#2" file.