Cut and paste this into a file, chmod +x it, and run it as
mpi_to_mpisim mpi_prog.c > mpisim_prog.c
----------------------------
#! /bin/csh -f

# converts mpi_init to mpi_init__

cat $1 | sed 's/MAIN__/main/g' | \
        sed 's/\(mpi_[a-z][a-z]*\)[ ]*(/\1__\(/g'                | \
        sed 's/\(mpi_[a-z][a-z]*_[a-z][a-z]*\)[ ]*(/\1__\(/g'