OK. Error messages for pages added by the hacker, you can ignore. Eventually, the search engines will realise that these pages no longer exist and stop looking for them. Anybody linking to one of these pages is not somebody you really want to have linking to you, so you don't need to worry about them.
For the pages which used to exist, ideally, you should set up a 301 redirect for each of the old pages to the nearest equivalent new page. If that's not possible, then your best course of action may be to set up a custom 404 page, explaining that the page requested no longer exists and offering quick links to the main areas of your site, so that people can easily find what they're looking for.
Submitting a new sitemap may also help to get your new structure correctly indexed by the search engines.