Have you considered taking tthe existing system back to the day it left the factory? You can find ,apk files from xdevelopers.com but it's a lot easier to reset the whole existing installation.
Swittch off and hold down the Voluume Down button when you press the Power button tro turn it on again. You will see a menu and, to make sure you really want to do it, you need to press the Volume Down button seven times to get to the reset option.
That starts the process and it wlll, of course, kill any personal data so it's best to slave the phone to your PC and back it all up.