The GEF code is a semi-empirical fission code that reproduces very well experimental data (much more than "classical" fission code). Could you add this model to GEANT4? You will find all the physics description and other details on this website: http://www.khs-erzhausen.de/GEF.html The existing code is written in BASIC (it may be downloaded from the website I have given previously) and is published under the free licence GPL-3 but I think the author of this code (Karl-Heinz Schmidt) may accept to modify the licence of his code if it prevents its inclusion into GEANT4.
I'll discuss this with our de-excitation model experts. It is a requirement that all code in Geant4 be written in C++.
You are right, it is require that to translate the code into C++. I know that GEF has already been translated in Fortran because the code was added in the TALYS code (http://www.talys.eu). I hope you could manage to include the GEF code in GEANT4 because it will be a great improvement for the fission simulation.
Further discussion will take place by e-mail as this is not actually a bug, but rather a proposed project.