From my point of view FNET stack can be used with MPC5604E, but author does not support FNET for the listed platforms anymore as Patrice mentioned above.
I easily ported fNET on MPC5744P. The MCU dependent files are well identified/organized. Take a look on this thread FNET - Embedded TCP/IP Stack / FNET User Forum / FNET User Forum:Import FNET stack to MPC5748G
Once the porting completed, you can add/share your files in the fNET. Of course the new one will not be supported even if there is no change. The author can't test fNET on all platforms.