That script does seem like a decent jumping off point.
The set theModifiedProjects line should probably be replaced with code that looks at all projects which are active.
The set theCompletedTasks line should instead be looking for tasks where completion date is false and due date is either blank or in that window between start and end.
Although I am not trying to make a script for OF to EN, I am trying to find out how to use AppleScript to create checklists in EN. Have you had an joy achieving that?