Thanks! It's a lot of smoke and mirrors to be honest - beyond the health alarm and the little tune for when the engine's on, the music's just a track playing in the background, so it'll add instruments no matter how far you've got.
The fuel tank's made using an AnimationPlayer. You can imagine setting it up so it'll empty over the course of a second as an animation, but instead of playing it, set the playback speed to 0 and use animation_player.seek(fuel_percentage) to update it every frame.
The score setting's done using a "singleton", which are nodes/scripts you can set to exist outside of the scene tree and which are not affected by loading/unloading other scenes.
My code's not the tidiest, especially not the stuff I was doing racing to meet the deadline, but I've uploaded the project file if you want to have a closer look at anything (and feel free to ask any more questions!).