If you have an idea which module / signal path is defective then it should be possible to get it working by doing some reverse engineering -assuming that the defective chips aren't custom ones made by Agilent for internal use-. Judging from the service manual, there is an extensive test suite that should bring you very close to the location of the problem. With an RF generator and oscilloscope / spectrum analyser it is possible to follow the signal path and see where it goes wrong.