By default drupal 7 find content page comes with filter by type and status. I wanted to give one more filter, where user can type the few letters from the title they are looking for so that they can find the article/page with ease even when theres 100's of content in the site.
I did something similar using views and exposing title as a filter. But it would be great to add this field and search to the default filters we find in the drupal 7 find content page.
Try the Administration Views module:
Replaces administrative overview/listing pages with actual views for superior usability.
It will let you replace the standard content admin page with a view, to which you can add as many filters as you need.
Use Find Content, simpler and newer than Administration Views.
Adds a search box to the Find Content screen
Requires: Node (enabled), Field (enabled), Field SQL storage (enabled)
I think it's admin accessible area and instead of modifying it why don't you try search module with Search Autocomplete module
hope it will help you :)