Unfortunately, you can't at the moment. Single missions and quicks are stuck at default but there is a way around the problem using 'notepad' and copying your preferred weapons loadout into the stock missions/campaign files (straightforward and works).
As you can see below, the stock single mission 'Bomber Intercept' in a Spitfire has tracers and a user defined weapons load out (which makes the Spit far more effective at downing bombers, etc). You can edit all the stock missions or even create variations using different weapons loadouts. If you want info how to do this let me know.
