If you accidentally deleted a system app or file like SMS, radio, contacts, phone etc from the /system folder and wish to restore it the here's how to go about it:
Note: This guide assumes the phone is still rooted, able to boot to home screen, install apps and you have apk(s) you'd uninstallled
Get the file(s) / app(s) you wish to restore
Copy the file(s) / app(s) to the phone's memory card