I should have more info about the app you need to be ported to to iOS so I give proposal to you based on a obscure set of info that I have by now.
I am a professional software developer for more than twenty years, focused only on iOS app development for more than a last five. During last year and so, my focus is on Swift apps which form the majority of my projects.
If you want some experienced developer that can easily read 3rd party code, understand and optimise it, I am your man.