That's a real nice bike you got there Senri. I also have a Nishiki (a Marina I think, it's from '81) which isn't NEARLY as original as yours. In fact, I only discovered it was a Nishiki when I converted it into a fixie a coupe of weeks ago...
Some 2 years ago I collected the bike for free and it had some bogus Concorde stickers on it, I never knew the real make of the frame until I stripped it in preparation of a rattle can paintjob. I did order some Nishiki stickers so it can proudly wear its colours again!!
Is your saddle a Kyosho? Mine came with a suede Kyosho saddle when I got it, I wonder if it's original. (it's got a yellow plastic BMX saddle now, hipster stylee lol)
About the bar tape, I'm tempted to say dye it to match the saddle for originality's sake, but I don't know how well the new color wil go with the amber brake hoods. As much as I like to see a close to stock Nishiki, a nice and sleek Brooks saddle would look awesome. I guess it depends on budget: if you're prepared to invest in a Brooks saddle go that route, if not just dye the bar tape.
EDIT: oops I didn't realise how old this topic was, got here via a link in an other topic