The Lockheed Corp. and the U.S. government tentatively have agreed to a $19.7 million settlement of court claims brought on behalf of 78 orphans now living abroad who were injured in the "Operation Babylift" crash of an Air Force-operated C5A near Saigon 9 1/2 years ago.

The agreement, signed Friday and made public yesterday in U.S. District Court here, marks the end of the majority of the lawsuits growing out of the 1975 crash, in which 135 people, including 76 orphans, died instantly as they fled the impending communist takeover of South Vietnam.

The claims of 52 children of American adoptive parents were settled in 1982 or resolved in individual trials.

Charles R. Work, the court-appointed legal guardian for the orphans, said as much as half the settlement is expected to go toward expenses and attorneys' fees. Each child is likely to receive $75,000 to $100,000, he said.

The settlement terms include $14.5 million in cash payments, $2.9 million in a trust for medical care and $375,000 for diagnostic testing of the children, who, according to Work, suffered brain damage when the plane's cargo compartment, where most of them were, decompressed explosively.

An Air Force investigation showed that the crash occurred after a lock system failed and doors blew off the plane at 23,000 feet. The pilot crash-landed minutes later on a rice paddy. The plane carried 330 people, including more than 200 orphans.

Lockheed earlier was ordered to pay $450,000 for tests for the "foreign orphans," the majority of whom live in France, with others in Canada, Italy, Sweden and other European countries. The manufacturer also paid $375,000 under a 1979 agreement.

Lockheed and the United States also agreed to pay an additional $390,000 to one plaintiff who won a $660,000 jury verdict here last spring. That verdict, now on appeal, would be left intact under the settlement. An undisclosed amount also would be paid in an individual case settled recently by U.S. District Court Judge Charles R. Richey.

An agreement between Lockheed and the Reagan administration setting the share to be paid by each defendant is under court seal. The Justice Department and a lawyer for Lockheed declined to comment on the pact.

"We are pleased that the parties concerned were able to reach an agreement that is both fair and reasonable," a department spokesman said. "We are confident that the settlement will adequately cover the present and future medical needs of these children."

The settlement is subject to revision and approval by District Court Judge Louis F. Oberdorfer. Oberdorfer indicated during a settlement conference yesterday that the agreement is likely to be approved with relatively minor changes.

"It's a great victory for the children," Work said. "I'm extremely gratified that this long ordeal is at an end."

Oberdorfer has complained that the cases suffered from "overlitigation" and predicted that the claims might not be resolved for another six years. The judge ruled that the Air Force had intentionally destroyed photographs of the crash site and "voluminous other evidence" pertaining to the accident.

In court papers filed yesterday, Work said the frustration of many of the adoptive parents with the American legal system was a factor in his agreeing to the settlement.

"By far the most important reason for accepting this settlement is that the children need treatment now," Work said. "Experts . . . tell us that for most of these children, the present years are crucial and time for treatment is running out."

Work said the children suffer from varying degrees of "minimal brain dysfunction" that left them with speech, learning and movement disabilities.