If that's not enough, this cheat should see you through the entire game. Whilst flying, hold down the left Shift key and type PLOP. This invokes a cheat mode making your plane invulnerable to missiles. What's more, if you crash into the ground the current mission will be regarded as accomplished and you'll be moved on to the next. Do the same to turn the cheat off again.
Gear damage could prevent you from either retracting or lowering your undercarriage. If you can't lower the undercarriage, land without it. Try to keep the VSI very low, and deploy the brake chute ( Alt-b) if you have one on touchdown. If things look bad when you hit the ground, eject.
If your wheel brakes are damaged, it will be harder to stop. Deploy the brake chute and air brakes (b). If you look like careering off the end of the runway, deploy your arrester hook (h). Every runway has arrester cables for emergency landings. If you can't acutally land on the runway, land near to it. You'll then be in for a rough ride, but should have a good chance.