Thank heavens for an alert motorist on the Orange Freeway. Could have been disastrous if he hadn't alerted others on the freeway to make room for an aircraft that had to make an emergency landing. It happened around 7 last night on highway 57, just north of the 10. John Findley, who was flying from Long Beach to Upland, says his Citabria developed engine trouble, and he knew he had to land the craft or else. When the motorist saw the plane coming down, he began swerving back and forth across lanes to keep other vehicles back, giving the aircraft room to land. It did. Close call, but luckily no one was hurt.