The Reagan administration has given Israel an unusually swift go-ahead to sell its Kfir jet fighters to Ecuador, the first such approval under a shift from past policy banning the plane's sale to other countries, American and Israeli sources say. The United States has a veto on the sales because the engines and other equipment are built under U.S. license.

American authorization, withheld under the Carter administration to prevent arms proliferation and protect potential markets for U.S. aircraft, is part of Washington's desire to offset Israeli displeasure at the approval of additional equipment for F15 fighters being purchased by Saudi Arabia.

The administration pledged a "much more forthcoming" attitude toward Kfir sales as it announced approval of the extra gear for Saudi F15s. But the switch also reflects what U.S. and Israeli officials describe as a generally more encouraging attitude toward sales of Israeli arms containing U.S.-made or -licensed components.

This is part of an increased belief in the State Department under Secretary Alexander M. Haig Jr. that Israel, regarded as a strategic ally in the Middle East, should be allowed to help offset its own defense costs by selling military equipment overseas, they say.

"There is no doubt in my mind that the Reagan administration is giving Israel an additional opportunity to sell some of the military equipment that Israel has to have," said an Israeli source familiar with such sales.

A U.S. official commented: "There is a feeling that it is not a bad thing to have Israel able to turn to its own resources for foreign exchange."

In addition to the Kfir, Israel hopes to increase exports of such military items as fast patrol boats, mortars and artillery, Israeli sources said.

These hopes have been buoyed by statements from Reagan administration officials to the Israeli government outlining the new policy -- and by the unprecedentedly swift response on the request to sell Kfirs to Ecuador. The application came back with a yes in less than 30 days, "the shortest time ever," an Israeli source said.

Bureaucrats who normally review such proposals in time-consuming detail at the Defense Department and the State Department apparently had orders from their superiors "to sign off first and review later," he added.

Given that experience, Israeli officials said they expect a favorable response on a pending request to sell the Kfir to another country in which they feel they have a chance, reportedly also in Latin America. They rate their chances of winning a contract with Ecuador as high.

The Kfir, a delta-winged fighter in regular use by the Israeli Air Force, is made by Israel Aircraft Industries, a government-owned corporation. Although praised for its performances, it has never been sold overseas because of the previous U.S. prohibition.

The needle-nosed fighter, patterned after France's Mirage, uses some American electronic equipment and a General Electric J79 jet engine, which although manufactured in Israel is made under a U.S. license and is thus subject to U.S. export controls.

Israel had arranged sale of the aircraft to Ecuador four years ago. But in one of its first major decisions on arms controls, the Carter administration vetoed the deal.

The U.S. refusal was based in part on desire to reduce the flow of arms and avoid an armaments spiral in Latin America. Israelis complained that it also stemmed from reluctance to see U.S. manufacturers lose business. They point out that even under the Reagan administration, the Kfir will be sold only when U.S. planes can compete.

In any case, former president Jimmy Carter partly reversed himself last October and permitted Israeli sales pitches -- still short of sales approval -- to Venezuela and Colombia. His change of heart came after a meeting with American Jewish leaders during his reelection campaign, Israeli sources pointed out.

Before that meeting, the Carter administration had turned down about 30 such requests to present the Kfir for sales abroad, including bids for Colombia and Mexico. It had approved presentation of Kfir only to Taiwan, which was likely to buy U.S. planes.

The approval for Taiwan came amid a search for ways to avoid offending China, which opposed U.S. arms sales to Taiwan. But it was precisely for this reason that the Taiwanese insisted on buying American and showed little interest in the Kfir.

Under Carter, an Israeli complained, "we could sell [the Kfir] everywhere they didn't want it, and we couldn't sell it everywhere they did want it."