Although I'm not familiar with the long format the simple answer is: if you don't already have a variable enabling you to select the cases you want just add one to your data file then use it to select cases.
under the Missing at Random assumption it is possible to model data with missing values so that you do not necessarily have to listwise delete those with any missing values indeed to do so could be problematic
This is a great site for discussing the issue of missing data with lots of advice