I did my best to keep it as close to the source material as possible, but my approach involved playing it on one computer while creating it on another. That led to a lot of reloading and trying the same thing making another choice in a question box. That definitely created room for error.
However, I'm playing through the Gog version of CotAB right now, and I'm running into some things that I know I never experienced before. So, I suspect there may be some version differences, as well.
Beyond that, there are some things that were programmed specifically for CotAB that never carried over into FRUA--the only examples I can think of off the cuff are in Pool of Radiance, but I'm sure that also happened in CotAB--so that's a third source of differences.
So, it wasn't possible to clone the game 100% accurately--this is totally fan fiction--but a lot of pains-taking work did go into making it as accurate as possible. And, you have the benefits of being able to use up and down arrow keys (rather than home and end), and the game remembers what spells you had memorized when you encamp...