I found a replacement nut that I’ve used for a FG-110 & FG-200. The stock nuts have a very narrow 1.36″ to 1.38″ string spacing (the center of E to E). The TUSC Graphtech PQ-6116-00 is 1.707″ long, 0.192″ wide, 0.331″ high, with a 1.435″ string spacing. That’s a huge difference in the string spacing. It’s not a drop in replacement, the bottom of the nut will need to be sanded to get the proper nut action. I got it on Amazon, for $11.60 at the moment.
Replacing the saddle is even less of a drop in. I’ve made my own saddles from Corian, “bone” colored synthetic counter top material. It’s very similar to TUSC. Since the vintage Yamaha’s have uncompensated saddles it isn’t too difficult. But it’s not simple, you have to sand to match the fretboard radius (or slightly larger) and round it over. And the bottom needs to be sanded flat & square and to the right height to set the action at the 12th fret. Many times you have sand the width also. A loose saddle will pull over forward from string tension, reducing its surface contact with the bridge, reducing volume. I make my blanks oversize and sand all of them to fit. I’ve found vintage Yamaha saddles are bone. Sand one and the smell will let you know. You can buy TUSC or bone saddle blanks and alter them.