Someone has tried to do it already, first with the original X-wing and then with Tie fighter. They were using the mission editor AlliEd, very good and highly recommended, but I cannot remember the URL. The PROBLEM is that LEC told the person who was converting Tie Fighter to stop. Apparantly such a project would interfere with their sales of XWlegacy, why buy that when you can dload the original xwing missions online etc...
In other words, you will have to make completley original missions or levels.