I was trying to make an infinite level. But I noticed almost everything needs to be set as a bullet to move to the left such as backgrounds and enemies (or right as you prefer). Specifically effects are problematic, especially when the effects are spawned by another object that is moving.
The spawned effect (like a missile smoke trail) is almost impossible to program because it needs to move to the left outside the screen, but also initially follow the missile.
I guess one option to avoid all this trouble is to make multiple ending levels, although I originally wanted to make an infinite level.
The player (which is an aircraft) also needs to move realistically with its own throttle and brake to a minimum speed. Something that doesn't seem possible with an infinite level, in turn.
Anyway, I guess I'm off to make multiple ending levels, but I'd still be interested if anyone has had any luck with what what I was trying to achieve. E.g. create an infinite level.