Redirect to a specific WordPress page instead 404

i tried with this code

<?php
	global $wpdb;
	$post_id = 30;
	$post_exists = $wpdb->get_row("SELECT * FROM $wpdb->posts WHERE id = '" . $post_id . "'", 'ARRAY_A');
	if($post_exists){
	  echo 'Sorry, you do not have permission to view this page.';
	} else {
	  echo 'Sorry but the page you are looking for cannot be found.';
	}
				?>

but this doesn’t solve the problem because it just checks if the post exists and it shows the proper message, anyway the post always exists.