I'm trying to convert an old AD&D module into a game, but I decided I would ask around to see if anyone has made some code for the core rules already. Why reinvent the wheel if you don't have to, ya' know?

Try this:
https://github.com/ThePix/quest/wiki/CombatLib

There are also some pages in the docs in the section Creating an RPG-style game:
http://docs.textadventures.co.uk/quest/

QuestJS has built in support, though at this stage it is a little experimental - and there is no editor, so you would be coding (but you should expect to be doing some coding in Quest 5 for this).
https://github.com/ThePix/QuestJS/wiki/RPG-Library:-Introduction

Thank you so much for the posts! I really appreciate it!

I re-invent the wheel every time...
It gives me better wheels that way.
So, what modules do you need?

I've got all the modules I need, but its just the backend code that I'm missing.