Use whatever editer you want and copy the section of the header.php file that is under the header comments, and place it in the header.php file of the forum.
Now you may also have issues with css, so check the <div ags to see if the forum has the same titled <div. For instance, if it has “<div #body” and the css also has the “<div #body” tags, then you may be able to use the template_css.css file for the forum, but I doubt that it will have all the tags needed. So what I would suggest is this.
In the header.php file for the fourm change the css file listed as required and replace it with the template.css. The body of the forum should keep the original css file and you will avoid some irritating issues.