Repair Help Needed

First, the truss rod isn't to adjust the action, it is to adjust the curvature in the first few frets.  The cause of high action is a bad neck angle, from 50 years of string tension pulling the neck up.  Only a difficult and expensive neck reset can fix that.  I don't know of an available drop in replacement truss rod.  I have a couple original ones, from scrap necks, but they would only be installed as part of a neck reset.  I've replaced one truss rod, as part of a neck reset, with difficulty it pushes out once the neck is off.  I wouldn't sell them separately, they are too valuable, I'd want to be sure they went into a project I completed.  Removing the neck is very difficult, depending on the year, the neck may be glued on with epoxy.  Without properly fixing the neck angle, there's no reason to replace the truss rod.

How does the fretboard and neck look?  The fretboard is typically difficult to separate from the top, removing the whole fretboard must have resulted in some gouging in the neck.

The lesson here is, never buy vintage guitars with horrible action.  Most of the time, properly fixing them is very difficult and expensive, typically costing more than they are worth.  Vintage Harmony's are easier to get apart and work on, mostly because the neck joints are sloppy and full of glue that softens fairly easily, unlike the "Asian Mystery Glues" that Yamaha used.

yamahavintagefg.boards.net/thread/224/inspecting-new-vintage-yamaha-fg

yamahavintagefg.boards.net/thread/18/vintage-yamaha-neck-reset-procedure

yamahavintagefg.boards.net/thread/685/benefits-neck-reset-fret-level

Do you know the measurements of the rod?

Thanks! I'll try to find one.

Not at all.  Search for luthier supply companies.  It would be doubtful to find an exact match, but you never know.