Convert huge (10-100MB of plain text) Mathematica function for external use

I have a huge Mathematica function (plain text ranging 10-100MB) that I am trying to convert to Fortran for external use. I tried using FortranForm, which does most of the work, and cleaned the code by hand using vim and some replacements.

When I try to evaluate the function, I get a Nan.
Here’s the problem: given the sheer size of the function, the compile-time already takes a long time and I can’t seem to be able to compile with debugging flags to see where the Nan originates.
My apologies for the vagueness of the question, but all input is welcome.
I also tried using MathCodeF90 but it seems to be too slow to generate that huge amount of code.