Look's like a good script.. I would possibly do it in this way and make it one on *:TEXT: event

on *:TEXT:*:#: {
  if ($1 == !ustats) {
    if (!$2) { msg $chan $+($nick,:) You have written $iif(!%lines- [ $+ [ $nick ] ],0,%lines- [ $+ [ $nick ] ]) line(s). }
    else {
      msg $chan $+($nick,:) $2 has written $iif(!%lines- [ $+ [ $2 ] ],0,%lines- [ $+ [ $2 ] ]) line(s).
    }
  }
  else {
    inc %lines- [ $+ [ $nick ] ]
  }
}

NOTE: This is untested.. Just thrown together.