The kernel is part of the boot.img. When you want to pull the kernel from a rom to reflash if you don't like a custom kernel you just pull the entire boot.img and replace the one in a kernel zip. Depending on the device, we'll use Samsung as an example cuz I'm more up to date on it, inside the boot.img are several files such as zImage(actual kernel is packed in here), intramfs, ramdisk, etc. But if you're just looking to reflash the roms stock kernel over a custom kernel to revert you just reflash the boot.img and it resets the kernel and all of it's associated files to the original that came with the rom.
☆SoA: Son's of Android™☆
I like to break stuff!