First and foremost, welcome to the forum.

Those JA Forbes magnesium reels seem to be well regarded, so it's worthwhile to find the parts and get it back on the water. They were reportedly assembled from parts made by JW Young, of England. I think you may be looking for the line guard or possibly a metal trim part for it. At any rate, your best bet could be to post on the Fiberglass Flyrodders forum in the Classic Fly Reels sub-forum, and of course, a good photo or two would help a lot. Good luck!