It is possible to replace the jets with flying classic cars. Something can be done in novice mode at level 6 that will result in a constant tone until you get to level 7. Level 7 will then start with classic cars flying at you instead of jets. The level passwords with the cars are identical to those with jets. You must duplicate some sort of game condition (complete all the first six levels with no damage? Complete level 6 with no damage and using only one plane?) to unlock the cars.
UFO's, not jets
When you get to the main screen, go to enter password. Enter level 21's password and if you hear a jet sound it worked. After that you can either start a new game or enter a password for missions 2-20. At the top, instead of migs it will say ufos: and then the number. When you start the mission, you will be battling ufos.