I need to install the missing air control flaps on my 1.8 L-jet. Is this possible to do without removing the engine?

I assume you mean the thermostat flaps that control the cooling air flow inside the engine tin? You will have to take the top tin off. That means removing the manifold stuff and distributor. Which can be done with the engine in the car, but it can be a real pain. The screws can be tough to get a screwdriver on, especially since (for me at least) the only driver that fit the heads properly is over 18" long...

--DD

Yes, it can be done in the car.