Fortunately, XDA Recognized Developer DooMLoRD has ported Sony Z Ultra GPe’s Android 4.4.2 KitKat to the Xperia Z1, giving Z1 owners a taste of what the pure Google experience is all about. Not possible without the kernel sources of the Z Ultra GPe released by Sony (kudos to them), the port runs on a custom kernel that’s a modified version of the Z Ultra GPe kernel meant to work with the Xperia Z1, as both devices share the same board.
As of right now, the port is up and running, although there are still a couple crucial functions that still need to be fixed, such as the camera and WiFi. But other than that, the majority of functions seems to be working, including phone calls, audio and video playback, 3G data, and Bluetooth connectivity.
Since this is a port from another device, one who dives in must realize that there will inherently be bugs and issues that may or may not be known to others, including the developer. The port is been tested on the C6902 and C6903 variants of the Xperia Z1, and only devices with unlocked bootloaders will be able to run the port.
If you would like to give the pure Google experience a whirl on your Xperia Z1, visit the original thread for more details.