There is a template file in /include/template called redirect.tpl. However I don't think it would be loading from style folder, if you copy one there ( but I may be wrong ). So You'll have to edit this global file.
What's more it's kinda hard to edit since there are only 2 lines of code. And putting anything after or before it, causes displaying it before or after main box.
So you'll have to build this from the beginning by including own files responsible for displaying header and rest of the text from lang files.
If any of my dropbox link fails with 404 error, change dl.dropbox.com/u/56038890/punbb/*.zip in address to 82283017.
Currently working on rPlus - responsive theme for developers to create their own themes basing on this one.