the ZXR was later but you can convert to ZX format easily. The gear diffs can still be bought, the ZX bodyshell is still being sold
http://cgi.ebay.co.uk/Kyosho-Lazer-Z...QQcmdZViewItem
The top deck and chassis doesnt really make a difference as the parts were available in 89. I think the ball diffs were as well.
The difference is so small between the zx and zxr (diffs, wheels and top deck), so nothing much to judge. The drivetrain is the same as the internal gearbox gears and belts havent changed at all.