I am a swift expert.
As a matter of fact , I have to deal with it - that, unfortunately, there hasn't been invented a program yet; the one which would help me convert your app to an iOS platform. Though there exist a tool( j2Objc - a Cross-platform applications) out there from Google but that translates Java code to Objective-C for the iOS (iPhone/iPad) platform. The major disadvantage of this tool is that it does not provide the developer with a platform-independent UI toolkit.
For better or worse, we have to develop a new one from scratch
Let’s get straight to the good news - the whole application ported from Android to iOS (new version of Swift) is going to be less costly than the first version was, so be prepared to spend the less money. The process you are going to save money on is the back-end development(If the Android app already have this) for your app - as you have already had it developed together with the first platform
Also, it is pretty possible the stage of specification writing is going to last less - likewise, it has already been written, so apart from some minor changes among these below crucial aspects the structure of your project is ready-to-go.
Navigation
Code and architecture differences
Design/UX
Device support
Frameworks
Customers and business model
If I can get the source code I could determine those APIs/Features that how do they work on swift and create a documentation accordingly included an approx. total Estimated Time of Arrival at least.