I had a similar (or opposite) situation with an '82 Honda Silverwing (GL500).  Perhaps it is almost the same thing you are having.  At about 150k miles it would pop out of 2nd on hard acceleration.  I tore the engine down and found a combination of two things.  The shift forks were slightly bent and the dogs that engaged the gears were getting rounded off.  I could buy new forks but the gears were obsolete.  I used a Dremel tool to re-square them up and it still runs great to this day.

I would definitely wait for a good answer or additional suggestions before proceeding.  If it is the trans, it may be easier to just find another used engine and swap them.  Otherwise, the parts could probably be found on eBay at a fair price.  It's a big job though.