It's not even that it's too hard. It's too slow. The rate at which you gain aircraft parts does not keep up with the rate at which you gain aircraft, other tech, or the rate your guys gain experience.
It's tedious to do 3-4 missions every time UFOs spawn. Getting access to basic torpedo planes a little easier could help cut out some of the tedium and also help jump start the player's early aircraft part economy.
It's hard to get parts from corvettes without any torpedoes.
Right now I have no troubles in my game getting alienum and alloys, I have shrikes, tanks, wave weapons, armors, etc, etc
My main squad is LTs or higher, but I've only managed to find 3 torpedo weapons.
My suggestion instead of having it so you can craft the parts is maybe to have the alien bases provide a good quantity of them. The alien UFO are always landing at the bases so it makes sense that spare parts would be in the bases too.