The rules state that the jam entry must be a "new game project".
I'm currently making a dungeon crawler project separate from the jam which serves as a study on the mechanics of the genre. Can I copy the code from there? Or do I have to code everything from scratch?
If hypothetically, I had made a dungeon crawler before, and copied the base mechanics from there to the jam entry, would it still count as a new game project?

If I were to make a prototype before the jam starts and build on it during the jam, would it still count as a new game project?

I feel that this is asked and answered. A major reason for attending a game jam should be to challenge yourself and create a game under pressure and hopefully learn something from it. 

Dunno if this'll help:

I write my project from nothing but source code. I don't use Unity or Godot or whatever.

So I regard the "you can use existing code" as stuff that brings me closer to what a base engine framework would give me, like rendering, playing sounds, accepting input.

I also regard things that help with building the world, but not specific to the game I am making for the jam. I pretty much always randomly generate a maze, and I have fairly well established maze generation code.

I also have a number of helper functions with random number/procedural generation. These are also not game specific, so I feel fine using these.

So, about to start in less than an hour, I will be bringing into my new project, which I will use Visual Studio to start in a brand new Github repo, I will pull over three assemblies that are not specific to the game.

These things will be first: a boilerplate game that builds, an itch page to host it, and a build script to deploy it semi-automatically. This is actually the goal for the first hour. 

I have pretty much done this each year. Haven't had a problem.

During the last week, I have been doing prototypes and whatnot to check that my "engine" has the facilities it needs to have in order to participate. There might be a line or two that I grab from here, but the prototype itself I will NOT be taking.