Re-install system Google Play store I tried installing a modded Play store using Lucky Patcher, but in the process it deleted the system app and didn't install the modded one.  I have tried installing it manually but it just can't work and crashes every time I want to download or update apps.
Can anyone help me out in installing the official Play store?
Just flash a GAPPS package available on xda. You can choose from a variety of packages, available on multiple versions of Android. See here for more information.