'91 has a wider rear subframe than the older ones. And bolted-on fuel tank rubbers, not glued on.
'90 and '91 also has different swingarm axle arrangement, with adjusting nut. '88 - '89 is more simple.
In a '90 - '91 it's possible to remove the rocker cover from an 1100 engine, in a '88 - '89 it's not.
Probably some more differences, can't remember now.