Lower level pair_styles in LAMMPS (like Stillinger Weber) are coded in C, which is not that different from Fortran. It is a good opportunity for you to pick up a new programming language. Unless the Axilrod-Teller potential is absolutely necessary to the work of any of the mailing list subscribers’, it is unlikely that anyone will volunteer for this task. Most of the LAMMPS features are contributed by users like yourself.
Cheers,
Ray