The kernel source is the same one found on www.kernel.org except for a small knoppix patch. Read the file /usr/src/knoppix-kernel.README for details.

You can download the latest kernel from www.kernel.org and copy the existing Knoppix configuration, then use "make menuconfig" to make small changes to the configuartion. There is a tutorial on that here: http://newbiedoc.sourceforge.net/system/kernel-pkg.html