I tried it! It's good for an alpha version!
I never played the original game, so I cannot say if you reproduced it perfectly or not. But regardless, here is a list of what I would like to be improved to make it better:
There are so many stars scrolling in the background, it makes me dizzy. Since the enemy space ships are also very dark, it makes it hard to see them. Less stars would help a lot. Outlining the ships or making them with a stronger contrast would make it better too. Oh, please reduce the speed of the stars. The background makes it hard to see enemy shots too.
2- Enemy ships
They really should shoot back. Boss ships do, but normal ships should too, even if it is less often.
It's a little weird that the player's ship changes direction when it reaches the end of the screen. I reached level 20 just by giving an initial "right" input and let the ship slide all along and jut holding "forward" to shoot.
4- Raise difficulty sooner
It's long until the game gets a little challenging. Maybe you could raise the game difficulty sooner and make the cannon upgrades more powerful to adapt to the game higher difficulty.
It would be better with sound, but since it's an alpha, no sound is good as well!
6- More enemy shapes and boss behaviors
But like I said it's alpha version so it can wait.
7- Missiles aiming
They don't move so well. It takes a while before they hit their target.
Now, what I really like:
1- Upgrades change the missiles amount
That is nice, although it's long before it happens.
2- You keep your upgrades when you die
That is so much better than the other shooter games that remove all upgrades when you die.
3- Authentic space feeling
Stars and space objects look very good.
4- Solid gameplay
I didn't run across any bugs or glitches, the games seems very robust for an alpha version.
That's it! Nice work!