Rear Master Cylinder Conversion

I am looking for some help (as usual! ) I recently purchased a new brembo caliper and smaller diameter rotor for my rear brakes on the MKII. I was told that I had to use a newer type master cylinder with a smaller diameter piston. Can anyone tell me where to find the master cylinder size of the newer Ninjas? I have spent hours searching with no luck. Has anyone converted to a newer style rear master cylinder? There are some really cheap rear master cylinders on eBay from the newer ninjas, but I'm not sure what year / model to buy. Any help would be greatly appreciated!
Thanks

I put rearsets on from a '05 zx6, and right side was complete...pedal, m/c, hose, and caliper. I removed the hose (too short) and caliper (wouldnt fit), used stock rear hose and caliper and had no ill effects, other than some time-consuming bleeding. Rear brake feels same as it always did.