i have Work with on Unity Games, i known of All Your Requirement how to do it & Also done it lots of time in our company as Your requrement like Bundle id changes, chage backgound, change design ,change appicon Also i known how to send push notification using onesignal , i can do it "Design new functions for the game like new boards where people throw knifes."
I will complete As you say that give the Knife Throw App a good upgrade with new functions