Congrats on your first Godot project, it's pretty well done! The graphics are quite nice, and the concept is also fun :)
The temperature/coal mechanics could be pushed further though, I found it relatively simple to throw some coal without overheating and then just go along. Even overheating doesn't really bring negative effects, you can't use coal anymore but you don't really need it as you're going full speed ahead. Clicking the bad guys is quite easy so they never got any of my ice cubes :)
Maybe the bad guys could come from more directions, with variable speed, and maybe have a way to cool down your engine (or the engine temperature could drop down faster by itself) so that you're forced to switch between pushing them away and reloading coal.
At any rate, for a jam it's a pretty good entry :)