I won't mince words: twin-stick shooter is a terrible fit for touchscreen. It's one of the genres requiring the most rapid, most accurate control
That's been a growing suspicion on my part - my best bet so far has been to add some form of auto-aim/aim-asist into the game to help make the game playable, much like Minigore did. Movement wise - I've spent quite a while working on making the controls feel as good as possible, but I'm still going to work at it.
Making interchangable ship parts isn't too bad of idea :-) tho I don't want to go overboard though on customizability and scare people away. Aside from power-ups and maybe different engines, what else would make sense to add to it without it becoming too much?