Unfortunately, the g77 Fortran compiler is no longer available in Linux. The best you can do is install libg2c0 using Synaptic; then you can at least still run your existing compiled binaries (if you stick with a 32 bit kernel).
If you want a Fortran compiler, you will need to install gfortran using Synaptic. To fix up your programs so that they run with this compiler, some hints are here.