You may be able to get the fitting from Porter’s (01243 377522) or Trident (0191 490 1736), both of whom specialise in Wayfarer gear and will know the correct fitting for your boat, although shroud plates are widely available if you already know the size etc. From memory you shouldn’t need to remove the trim from under the side decking, although I’m sure boats from different builders vary on details like this.
The part that remains on the surface of the deck is (as far as I know) just a flange with a slot that the shroud plate passes through. The lower, intact part of the broken plate is probably bolted to the inside of the hull just above the waterline. The plates seem to be supplied in two parts (see picture) – presumably the shorter length is a backing plate sandwiching the hull – without a boat here to look at I can’t remember exactly 😆
It’ll be obvious once you start investigating properly. Good luck.