The site here contains the most up-to-date instructions to install Kismet drone on the WRT54G series
It suggests that:
" If a build with the Linux 2.6 kernel is available for your hardware, you should use it."
It took me after a while to figure how to upgrade OpenWrt from linux kernel 2.4 to 2.6, luckily it turned out that everything is already in the wiki page on OpenWrt website, but not very obvious.
What you'd need to do to upgrade to kernel 2.6 is to upgrade OpenWrt firmware image by just flashing on the LuCI web GUI with this Broadcom47xx image:
Now I have 2.6 kernel with Kismet-drone installed, with 232.00 KB left in the Flash ROM.
[Note: I have tried to run Kismet drone on TP-LINK TL-WDR4300 today, so far no luck. Possibly I still don't know how to configure the ncsource in /etc/kismet/kismet_drone.conf]
Any suggestions/comments would be much appreciated.