My suggestion is this...
if you still have your old kernel set up as "Linux.old" which is the default when you have compiled a new kernel...
do:
grep eth0 /etc/modules.conf.old
this *should* give you a line like:
alias eth0 <module name>
so you should get to see the name of the module the old kernel was loading for the network card (which it called eth0).
once you know that you can do:
grep <module name> /usr/src/<old kernel source dir>/drivers/net/Makefile
and the *easiest* way in the world for you to fix this up is to simply copy that line into the Makefile in your new kernel source. (but make sure that the line does exist and is just one single line first)
ie:
grep <module name> /usr/src/<old kernel source dir>/drivers/net/Makefile >> /usr/src/<new kernel source dir>/drivers/net/Makefile
NOTE THAT THE REDIRECTION IS ">>" AND NOT ">". the first one tells it to APPEND to the file, the second tells it to clobber (overwrite) it!!!
alternatively, just do a 'make menuconfig' and pick the module under network cards (10/100)
so there you go, you should be ok with that I think, unless the module isn't aliased in your modules.conf or whatever.
in that case just use dmesg, or else boot into the old kernel and try 'lsmod' to see a module listing.
good luck
(and P.S. please don't flame me for telling someone to mess with their kernel source files like that - if he/she does it properly it should be fine.)