Paul,
Page 10 of the Pearson 1980 Ensign Parts Catalog, which can be found on the ECA website under Library/Historical Documents gives the diameter of the shrouds, forestay and backstay as 5/23”. They also give the turnbuckle size as 5/16”, which may be useful for determining the size of the turnbuckle boots.
However, you can mave be certain that a previous owner did not install a non-standard diameter shroud, and 5/16” turnbuckles from different manufacturers may have different outside diameters, so I would recommend checking your boat first.
Vic Roberts
#2032