my rickety old self couldn't take rigid, even on some high volume tires and a flexy ti post...so i went squishy on my '99. the original superfly fork on the '99 was 75mm, so stay close to that and you won't screw up the handling (or the aesthetics) too badly.
i had an 85mm reba on an old steel frame that was designed for no suspension and it totally ruined the ride, so i moved it to the KK and i love it. +10mm is not noticeable in the handling.
and i can't say enough good things about the reba. plush, tuneable, plenty stout. not exactly period correct, but a great fit. a little heavy.