There seems to be few differences on the 250z and its earlier brothers, to note a few there is:
nylon chain buffers, upper and lower, on the chain guide
different fork innards (these are 250x forks fitted in pic-originals come with the bike, but chrome is pitted around the triples)
different part# on rear shocks
chain gaurd at front sprocket is different
tail light (although I have never seen what this should look like)
levers- dog leg type?
foot brake has folding tip (original comes with bike, but has broken spring)
cosmetics ie. different graphics
possibly a few more differences, but they dont really jump out at you.