You would need to add category to the SQL first
So
FIND
if (!$pun_user['is_guest'])
$result = $db->query('SELECT t.subject, t.closed, t.num_replies, t.sticky, f.id AS forum_id, f.forum_name, f.moderators, fp.post_replies, s.user_id AS is_subscribed FROM '.$db->prefix.'topics AS t INNER JOIN '.$db->prefix.'forums AS f ON f.id=t.forum_id LEFT JOIN '.$db->prefix.'subscriptions AS s ON (t.id=s.topic_id AND s.user_id='.$pun_user['id'].') LEFT JOIN '.$db->prefix.'forum_perms AS fp ON (fp.forum_id=f.id AND fp.group_id='.$pun_user['g_id'].') WHERE (fp.read_forum IS NULL OR fp.read_forum=1) AND t.id='.$id.' AND t.moved_to IS NULL') or error('Unable to fetch topic info', __FILE__, __LINE__, $db->error());
else
$result = $db->query('SELECT t.subject, t.closed, t.num_replies, t.sticky, f.id AS forum_id, f.forum_name, f.moderators, fp.post_replies, 0 FROM '.$db->prefix.'topics AS t INNER JOIN '.$db->prefix.'forums AS f ON f.id=t.forum_id LEFT JOIN '.$db->prefix.'forum_perms AS fp ON (fp.forum_id=f.id AND fp.group_id='.$pun_user['g_id'].') WHERE (fp.read_forum IS NULL OR fp.read_forum=1) AND t.id='.$id.' AND t.moved_to IS NULL') or error('Unable to fetch topic info', __FILE__, __LINE__, $db->error());
REPLACE WITH
if (!$pun_user['is_guest'])
$result = $db->query('SELECT t.subject, t.closed, t.num_replies, t.sticky, f.id AS forum_id, f.forum_name, f.moderators, f.cat_id, fp.post_replies, s.user_id AS is_subscribed FROM '.$db->prefix.'topics AS t INNER JOIN '.$db->prefix.'forums AS f ON f.id=t.forum_id LEFT JOIN '.$db->prefix.'subscriptions AS s ON (t.id=s.topic_id AND s.user_id='.$pun_user['id'].') LEFT JOIN '.$db->prefix.'forum_perms AS fp ON (fp.forum_id=f.id AND fp.group_id='.$pun_user['g_id'].') WHERE (fp.read_forum IS NULL OR fp.read_forum=1) AND t.id='.$id.' AND t.moved_to IS NULL') or error('Unable to fetch topic info', __FILE__, __LINE__, $db->error());
else
$result = $db->query('SELECT t.subject, t.closed, t.num_replies, t.sticky, f.id AS forum_id, f.forum_name, f.moderators, f.cat_id, fp.post_replies, 0 FROM '.$db->prefix.'topics AS t INNER JOIN '.$db->prefix.'forums AS f ON f.id=t.forum_id LEFT JOIN '.$db->prefix.'forum_perms AS fp ON (fp.forum_id=f.id AND fp.group_id='.$pun_user['g_id'].') WHERE (fp.read_forum IS NULL OR fp.read_forum=1) AND t.id='.$id.' AND t.moved_to IS NULL') or error('Unable to fetch topic info', __FILE__, __LINE__, $db->error());
Then just use $cur_topic['cat_id']