hi Ri2do, i give u some hint
when the next update from miui release (download miui for n1), u can used my rom as a based system
also visit miui china and download the ota update to see what file that has to be update the system.
just open my zip file using 7zip, delete the app folder from my zip file and then drag n drop app and framework folder from n1 to my zip file
it will boot to the system and give some bug (sometimes boot stuck, light sensor, etc)
to fix the boot problem you should editing services.jar (it's only be done by trial n error), light problem is editing services.jar and framework-res.apk use this link as reference to editing the file
another bug, just read the logcat to find the bug and solve the problem
i hope this will help you to build your rom