valve springs and retainers, get the rev limiter raised, change the cams, deck the head, rods/pistons, port/polish, full exhaust system, velocity stacks, get a reeeeeeeally good tune on the bike.
drop a tooth on the front sprocket, add 2 on the back. put in Rifleman's 1/5 throttle insert.
Also, get brakes. much better brakes. and suspension.
Brembo brakes all around and Ohlins/Racetech or the like for suspension.
then buy a full leather suit, gaunlet gloves, race boots and buy a few track days.
08 ZX6R Radial M/C ZG Sport Touring Windscreen CBR600RR Calipers BMC Street Filter
GP Shift Pattern DIY Mod Fender Eliminator Kit 16/44 Gearing Euro Flushmount Signals
Gauge Cluster Color DIY ModSoupy's Fully Adjustable Lowering Links HEL Stainless Steel Braided Brake Lines