For most of the cases, open Display.controller.php, search for:
'reply' => array('test' => 'can_reply', 'text' => 'reply', 'image' => 'reply.png', 'lang' => true, 'url' => $scripturl . '?action=post;topic=' . $context['current_topic'] . '.' . $context['start'] . ';last_msg=' . $context['topic_last_message'], 'active' => true),
and replace it with:
'reply' => array('text' => 'reply', 'image' => 'reply.png', 'lang' => true, 'url' => $scripturl . '?action=post;topic=' . $context['current_topic'] . '.' . $context['start'] . ';last_msg=' . $context['topic_last_message'], 'active' => true),
It will show the button even without permission to reply, when someone clicks on it will ask to login or register.
There is the side effect that if the guest registers he will not be redirected again to the topic and the reply forum (because "of course" he has to get the email verify the account, etc.).