FL Studio Mobile runs on every device with iOS 5.1 and higher.
If your iOS device can not run iOS 5.1 or higher, it is not possible to reinstall your FL Studio Mobile app.
A hardware update is needed in that case. As soon as you have new hardware, and the iOS can be updated to 5.1 or higher, you regain access to your earlier purchased app in your iTunes account.
The soon to be released FL Studio Mobile update (expected in September 2014) will be optimized for iOS 8.