Google official factory images of ALL NEXUS devices
just add 'https://' as i was unable to post links
Form the list select "Factory Images "nakasi" for Nexus 7 (Wi-Fi)"
you can find links of 4.1,4.1,4.3 and 4.4
the extension is not 'zip' it is 'tar' so flash it using 'Wugs toolkit' which is awesome!!!!!!!!
IT is the first thread in nexus 7 Android development
IN wugs toolkit there is option 'Back to stock' - flash stock + unroot
after flashing you will need to root again which is a piece of cake using wugs toolkit
That guy deserves a huge appreciation and thanks