How to mount system in rw mode if no custom recovery
I would personally use ES File Explorer to get the original build.prop back into your /system partition. You have to go into the app options and select "mount system read write" and it will be able to write to the /system partition. 

The commands for mounting the partition and copying the old build.prop from the root of your sdcard are:

$su
#mount -ro remount,rw /system
#cp /sdcard/your_old_build.prop /system


You don't need to use those though if you are using an app that gets root access.