I know what you talking about. You're referring to Pokémon Essentials GS, right? Well, most of it are graphical changes or sound changes to mimic the Gen II limitations, the core of the code is pretty much the same as the version 13 of Essentials. Honestly, it would be easier to do as the told above and make the changes on a version 18 of essentials, but this would demand some work and patience, plus, some knowledge of ruby.
The best way to start this project, in my opinion, is to answer the following question: Do I want to make a genuine recreation of the Gen II Style, or would I take some creative liberties?
In other words. Do I want to make a fully immersive Gen II experience, with the same limitations as the GBC games or would I be willing to make some changes. For example, the aspect ratio of the screen. If you're going to mimic the GSC games, the screen should look in the 10:9 aspect ratio (that would make 160px width per 144px height), except that you don't need to make it this way.
Super Pokemon Eevee Edition is an example of a game the clearly takes inspiration by the Gen II style, but doesn't let it restrains by it, so the aspect ratio it's not the same as the one used in the Gameboy Color hardware.
Aside from the fact that this fangame was made in a different engine, this shows the importance of making these kinds of questions early on in production, or preferably before it. The "square" ratio would be more faithful to the original style, but the "rectangle" ratio would allow more information to be display in the screen, thus making some mechanics such as double battle more viable.
Plus, it would be much more easy to convert the engine to this style, cause you wouldn't be worried about the placement of information, that necessarily would need to be changed, in order to the GBC aspect ratio work. This can be done somewhat easily, if one's have patience enough to go around the Essentials script and change the lines of code that defines those informations. But again, this would be time consuming and someone unfamiliar with ruby would be lost in the beginning, trying to work around how to make the changes that you want to see happening.
Now, what wouldn't be possible is to 100% update the original Pokémon Essentials GS from version 13 to version 18. If changing every single graphic and information placement sounds time consuming, this would be 4x worse. From then to now, many changes happen, now only on how something were written (such as DEFAULTSCREENWIDTH becoming SCREEN_WIDTH, for example), but the way that the code interprets some mechanics such as the battle engine have changed a lot. Now, I'm not the best programmer out there, or even a programmer, so to speak. I'm just a guy messing with some lines of code, hoping that eventually I've discovered how to make what I wanted to make possible, either by accident or not, but I've tried to experiment with some older scripts to see if I could work around it, but most of the time it doesn't go well. Sometimes it's something simple as change DEFAULTSCREENWIDTH to SCREEN_WIDTH, sometimes it's something so complex that I can't understand what have changed from then to now.
That's why I advise to you to think what is best for you. Would you to venture into the Script Editor and try to make the necessary changes yourself, or would you prefer to stay with the more stable older version (I mean, the Pokémon Essentials GS version)? Do you want to set your project with the limitations of the original GSC games, or do you think that some changes are welcome (for example, the use of six channels instead of only four, as used by Gamefreak in the GB Sounds mechanic in Heartgold and Soulsilver)? What would be the best way I can work around to convey my ideas through the tools available to me (the Essentials, the GS Essentials, Gen II style, etc.)?
Once you get these answers, you can start to picture what is viable and what it's not, and from there, start to work on what you think it best suits your project.