Generally good - improvements possible
It's a fun game to waste time with and the custom ship drawing is a great idea, but I have a few suggestions.
Firstly, an inertia system? Like if you're in a real spaceship or something, and turn hard, you'd swing round without changing your movement heading. I'd like that for tracking the enemies easier. At the moment if you turn at high speed you still get perfect on-rails grip and make a U-turn.
Also the bullets are a bit slow, imo. It's frustrating to have to devote so much attention to leading the enemy with the bullets.
Lastly I know it's a doodle ship you're meant to be flying but you know mice. How about a line tool, or a togglable snap grid or something.