[Q] Kernel

Hi,

i have several kernel related questions.
The 9300 has 4.0.4 stock with an insecure kernel flashed.
I did that in order to use s3 tool kit. Now i wanna go back to
original stock samsung kernel.
Where do I get the right file to flash with Odin?
What is the biggest mistake i can do?
Will everything work like before or will I have to reinstall stuff?

thanks