Not sure about one as old as yours but all the Nacras I've seen use a nylon ball for the mast to rotate on. When it starts to flatten out on top, replace it. It's threaded, don't overtighten! A pin holds the mast step captive to the ball and the mast must always be turned ninety degrees when raising or lowering.
Shrouds are the wires that hold the mast up. The front one is usually referred to as the headstay or forestay. The diamond wires are part of the mast and are used to limit mast bend.
If you want/need new standing or running rigging, If you wish to support local (Central FL)
you can call my friends at The Yacht Rigger in st pete. talk to them and you can send them your old side&fore stay/diamond wire and a bridal wire (1 each) and they can match them for you