So much potential..
This game shows a lot of promise..
Making a custom spacecraft is totally awesome.. except that it has no significant effect on gameplay. The game would be way cooler if it did. Could the player get those multiple part spaceships like the bosses?
Bosses with multiple hitpoints were pretty cool.. except that successive bosses didn't actually become more powerful.
Having a scrolling, dynamic tunnel is neat - except that the tunnel doesn't get thinner over time, it doesn't scroll faster as time goes on, and it doesn't hurt to crash into it.
Also, I'm not a fan of having unlimited shooting ability. By clicking my mouse and my keyboard's mouse button at the same time, I can get totally crazy bullet counts with the normal gun. If you want to do a retro theme, the usual way that old games handled that was to allow the player to only have some certain number of bullets on the screen at once.
Decent job, overall. If you go through the extra effort to cash in on some of this potential, you could easily have a 4+ game here.