As far as I know, there's no way to manually re-add the default Google search with all its features.1
However, you can delete the configuration file that contains user-defined searches and let Chrome recreate it.
Locate your profile's Chrome folder:
Windows 7 and Vista:
%USERPROFILE%\Local Settings\Application Data\Google\Chrome\User Data\Default
Mac OS X:
If you're not using the default profile, replace
Default with your profile name.
Move the files Web Data and Web Data-journal elsewhere (so you can restore them if their absence causes unexpected problems).
Reopen Chrome and sign in (if you use a Google Account).
Select the restored default Google search in Manage search engines and make sure the option Enable Instant for faster searching is checked.
1 The default Google search doesn't appear in the Web Data like user-defined searches. Selecting the default search modifies the properties
"default_search_provider" in the Preferences file. Even modifying those properties manually doesn't have the desired effect, since Chrome removes them on restart.