It's quite likely that there will be -- 2.4.20-8 is a vendor kernel. I'd go to www.kernel.org
and fetch a 2.4.23 kernel tarball. Unpack it somewhere (/tmp and your home directory are good spots), and follow the instructions to configure it.
Once you've configured it, download the forcedeth patch and save it into the root of the kernel source (i.e. the top-level directory where all the other directories are). Once you've saved it, go to the top-level directory and execute this command:
cat forcedeth.patch | patch -p1
Once the command finishes (hopefully without errors) go back to the kernel configuration system and go to the Network drivers section. Fin the forcedeth option (CONFIG_FORCEDETH) and select it as a module. Save, exit, compile and install your new kernel.
If you need more assistance, don't hesitate to ask -- but be warned that I may not be able to explain it as clearly as some of the kernel-compiling HOWTOs available on the Internet.