Sounds terminal unfortunately, Usually nasty noises from the transmission require a strip.
Could be a selector fork or a Circlip come adrift on one of the gear trains allowing the gears to shift out of sink.
It's worth looking at the shifter mechanism as there are various parts that could cause your problem..When you shift into 1st the mechanism moves one way then the other for all other gears, Also there are some springs in there that could have expired causing you grief.
Kawasaki use what they call a positive neutral finder and i think they were used on your model if memory serves me and this could be one of the causes for your problem....
There are three small balls on the output shaft that due to centrifugal force are thrown out allowing second gear and up to be engaged but only when the output shaft is turning 'Ie' when the bike is moving..
One fault with this system is that you are only able to engage 1st and of course neutral..
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