you can forget about Marzocchi bushings .
i had a friend who had the same problem with his 40 mm KTM forks
i did a little searching on Google and found that a few Yamaha road bikes had 40 mm Kayaba
forks with upper bushings in the fork slider ( V Max and a few others )
i never ordered some at the dealer so i could not compare dimensions .
the Yamaha part number is 3JJ-23125-00 . you might have to machine the slider to accept the Kayaba bushing .
if you go this route please let us know how it did work out
Eldert