External/ Custom 404 message

So a friend gifted me this 404 page layout:

http://themeforest.net/item/uerror-custom-error-page/2460996

I’ve been playing around with it and now have this “404 page”:

http://www.katharinasabel.de/404 (just an index.html in a folder)

My question would now be, can I even use this as my default 404 page in Mantra? I can only find the 404.php file and I’m not good enough in .php do know how to redirect users to something else. Also would there be a way to integrate the search function of the 404 page with the Mantra search? Using a searchbox in the .php file is really easy, can it be translated?

Thanks in advance. I know it’s kind of a strange question as it has only limitedly to dow ith Mantra. I still hope you can help me because the 404 page looks so awesome :3