To modify the feed for Yandex Market that was not unloaded in the price of the missing stock items

The website runs on MODX. There is a fixed module unloading in Yandex Market, but he unloads all items in a row, even those which dont, and you need to make missing products in the feed has been removed, i.e. to prescribe meaning to them. Extra TV-the field responsible for the availability of my site is p_nal, but for each of the two options, namely "available" and "Not available" values are not spelled out, screenshot attached. Therefore we need a solution to this problem.