Wow, this is great! Battlezone is the perfect example of an old-school cockpit game, and this is really smooth and responsive. It would be really cool if you expanded on it a bit.
Have you played an old C64/Atari 800 game called Encounter
? In my mind, it's one of the best Battlezone clones, and part of the reason for this is the audio. It's all electronic beeps and warbles, but each of the enemies has a distinct sound that you use to determine what kind of threat you're up against, and whereabouts it is. The noise that the missiles make as they speed towards you still quickens my pulse to this day!
If you were to take some ideas from Encounter, I'd suggest using audio for the enemies, getting louder as they get closer, and perhaps making use of stereo. Seeing as you have multiple enemies, perhaps each one could have its own pitch, assigned at random. Also the simple radar makes figuring out if enemies are behind you a lot easier, so you get shot in the back less.
Just some ideas for if you expand on the game, anyway. I can help you out with sound effects too, if you need them.