Hi, I have a 2003 kawasaki kfx400, with a full leo vince exhaust system, k&n oil filter and k&n air filter. Im thinking of going for a big bore kit bur i would like to know what should i replace when replacing to a big bore kit?
Thank you

 

Welcome to the forums.
I'd contact a motor builder such as: Baldwin motorsports/Jannsen motorsports/Curtis sparks racing. These guys built a ton of these Suzuki's in the 400 days.
FYI: the 416/426 kits were the most dependable.
IMO, If you're looking for speed/power I'd stay stock and go with some porting and a cam.