Im a little surprised no ready options exist for simulation. What's your precise goal? Stable flight? Programmable motion? Self steering?
If this was a project I was doing, knowing nothing about flight, here's the general shape of how I'd give it a shot with AI:
Capture data set from real controlled flight, sensors, input, output, as much as you can get.
Use this to train a model or heuristics so that you can predict eg sensor state from input. You have gyros or something I hope?
Iterate here as needed, as you find weak spots. Keep collecting data.
If you have specific goals (navigate some environment, etc) you'll probably need more pieces, of course, this is just basic control loop stuff.
Once you can do this reasonably well, you now have a rough proxy test for actual controlled flight, and can iterate on controller via simulated flight.
Give codex specific quantifiable hill to climb in sim (stable for X time, navigate these environs, etc.)
If results work well in sim but not in real flight, work on your data pipeline some more, improve sim.
But I'd spend time looking for existing sim resources first. This seems like a problem that's others will have solved at least some parts of already.