You don't need to build an actual kernel. Just install the kernel-headers package:
apt-get install kernel-headers-2.2.20 (or something like that)
After that you likely need to make a symlink to /usr/src/linux and the kernel module should compile.
Something else I would upgrade to some 2.4.x kernel (debian ships with one too but default is 2.2x...) since it can be a big improvement.