Hi Jack, welcome to Ninjette!!!
Easiest rear-shock option is to pick up 1st-gen or 500ex rear shock and replace spring.
They both are direct bolt-ons with removable adjustable spring-collars. See my
shock comparison thread.
Also get shock-linkage knuckle from new-gen, the leverage-ratio was changed to move shock more for same wheel-travel amount, giving it more leverage.
As mentioned, Sonic springs for fork, or
KZ1100 springs as someone else has tried.