I had both, here are the differences my memory can remind from my racing days back in 1989-90
as anyone can see, different body
different wing and wing support
zx had 1.9 wheels zxr 2.2 wheels
zx had gear diffs zxr ball diffs
zx had short gold front shocks zxr had medium size front ones (developed for zxr and triumph 2wd)
zx had belt plastic cover zxr had no cover
zx had a split upper chassis plate, zxr was one-piece
zx had a third center diff in the 100 T crown , zxr had a slipper clutch in the 100 T crown.
zx had alloy shock towers zxr had fiberglass different shape shock towers.
zx had no hex adjust turnbuckles zxr come with them
the upper front suspension link in zx goes in different mounting point. much simplified in zxr
I think this are the major differences, but there is also some of details for "anorak's" like a pivot advance change in the steering geometry in some zxr and different chassis materials in very early zx (earliest zx chassis was graphite instead of frp. I know it because mine was a very early kit, bought in 1989 some weeks after press release. )
another detail, the 100t gear pitch was different, zx was metric, zxr was 48 pitch, a subtle difference, but pinions does not interchange perfectly .
hope this helps.