what to do with fighters once ufo mission complete

As per title, do I sell fighters now I have completed ufo mission or is there a way of saving them to warehouse to free up space for a jumbo?

The game doesn't allow you the option to move items to the warehouse. If you have maps to Area 51, you can still use the fighter jet to fly there. Keep the fighter jet if have the hangar space, otherwise you have no option but to sell it - just means that when another mission comes up that needs the fighter jet, you'll have to complete the collection to get another one.