SexMax is only good for the Evo class in Australia - being a 2013 model, it gets bounced out of all of the VMX classes that have a cut-off date.
Dunno how the Keewees do it, but in Australia the Pre-90 class is for 1989 models and older - so it needs to be a carry over of a 1989 model, not a carry over of a 1990 model.
Is there even a Pre-90 class over there?
Without doing any research:
Forks are visibly different;
Bodywork is different;
Motor is similar - no idea whether its close enough to be called a carry-over.
If the frame, swingarm and linkage were the same as the 89, then a 91 model fitted with 89 forks and bodywork, would probably be a "close enough" bike. I wouldn't take it to an Aussie title meet, but I've seen plenty of less-legal bikes being happily raced at club events without an eyebrow being raised.