This game has been a good reference for pixel artists, due to the great color palettes used, but in my humble opinion, it wasn't that good as a game. I played it in my SNES, two players mode with my brother, and it was a bit boring.
Also note that this game was designed to be played with keyboard or gamepad. On screen controls trying to emulate game pads were, are and will be uncomfortable, no matter what you do, because you can't feel the contact with the buttons and often lose the reference. Anyway, maybe the best device to deploy on screen controls is the ipad mini, due to its medium size. I prefer fixed position for the controls, by the way.
Where are the buttons now? It has no sense to make them transparent if there are more than one button. Also, it would be good to have the possibility to hold the button for continuous fire. Another option is to make auto-fire when you have an enemy ahead.
Even if you cannot use the graphics, sounds and music, you have a game engine, so you can try to make an original project from scratch.
Good luck with this!