I found a source for replacement pick guards for the FG-300. The celluloid FG-300 pick guard is known for degrading and crumbling, typically from being stored in a hot location. Many times it discolors the finish on the top and oxidizes the frets near it.
Backwood Guitar in Sedalia, Missouri, started by copying one from an original pick guard. Since then, the demand has grown. They are all made by hand in small quantities. The graphics are silk screened and covered with 2 coats of lacquer.
https://reverb.com/item/18115878-yamaha-fg-300-replica-pickguard-free-shipping
I’ve also made a PDF template of the pick guard from my FG-300. It is available HERE.