This is the bit from Staff/Viewticket, it should give you the options.
<?if(!$ticket->isoverdue()){ ?>
<option value="overdue" <?=$info=='overdue'?'selected':''?> >Mark Overdue</option>
<?}else{?>
<option value="notoverdue" <?=$info=='notoverdue'?'selected':''?> >Un-Mark Overdue</option>
<?}?>
just need someone to write the class.ticket bit and scp/tickets
for notoverdue