Here we go about the springs for the valves
The data I show here is coming from the strongest ever build
250 Ninja, a real race bike, with 45.4 HP @ 12418 rpm. With all the other given restrictions this bike revs up to 13500 rpm.
The hp is given at the crank since an engine normally at first has to be seen as a standalone device and a real engine build is made on a workbench and not inside the frame of the bike (sorry but since to many talks about this issue I couldn't hold against myself, so if one doesn't like my direct way to talk then you can rate this as negative helpful).
The parts are from the ZX6R and listet below:
Valve-Guide: 49002-1152
Retainer Valve-Spring: 12009-0020
Collet: 12011-1052
Valve-Spring: 49078-0013
Seat, Spring: 16007-1227
Seal, Oil: 92049-1398
Tappet: 12032-0014
Good luck for everyone in tuning his bike
PS: I forgot the link and the PDF:
http://www.scuderiaplatini.it/250_racing.html