Decompile Apk Game App (500MB) and Add some features

Hello, my name is Khoa, I have some request for Android/iOS developers Background: I have a ready-made game source. However, it does not use MySQL and it does not have a working Login/Sign panel (it just has a button "Play Demo") The database of the game is saved to files (not MySQL tables) What I need is: 1. Decompile and Integrate the Login/Register panel into the App. (It needs somehow connect the new Account database to Character database) 2. Analyze the database and find a way to access into it. I tried to open it with Notepad++ but didn't work. (However, I have some clue. I have the web [login to view URL] which somehow can access to the data, it can add money and items to specific account, but it does not have tables and stuffs) 3. Decompile and Integrate the payment system into the app. I do have the APK and IPA files already. If my requests are vague to you. Feel free to contact me and discuss more.