I have extensive android and iOS experience, as well as doing 3d unity work at my current company, where I have been working for the past year. However, I am looking to move to freelancing so that I have more control over the hours I work and vacation. I am willing to work overtime as needed by a project, but mostly I just want an interesting challenging project. I am a little confused because your posting says the iOS version is already available on the app store? is the video processing done local to the app instead of on an app server? if the iOS is done, don't you just need someone who can do both iOS and android so that they can port your app? I am skilled at both and could port the entirety of your app from iOS to android if needed. Or if a server is processing the data, it sounds like you would just need to duplicate the calls.
However, even if the iOS code cannot be reused, it doesn't sound particularly difficult to create overlays on a video file. I would need to take a look at exactly what is needed to give you a time estimate, but I would be willing to spend a weekend day looking it over free of charge to determine how long it will reasonably take.
Sincerely,
James Street