I want to (un)modify the files manually.

You can! All you need to do is edit three files. Here's the code to add (or remove).

Filename	Code
/index.php	before "//Installation Subfolder Check" add require_once( 'components/com_page_cache/page_cache.class.php' ); Before "//Detect First Visit" add global $pageCache; $pageCache->init(); ob_start("jos_page_cache_store"); Before "doGzip();" add ob_end_flush();
/administrator/index2.php	Find "$my = initSessionAdmin(...);" after, add require_once( '../components/com_page_cache/page_cache.class.php' );
/administrator/index3.php	Find "$my = initSessionAdmin(...);" after, add require_once( '../components/com_page_cache/page_cache.class.php' );