Introducing a note of optimism suggesting that a negotiated settlement to the Syrian missile crisis can be reached, Prime Minister Menachem Begin said tonight after meeting U.S. special envoy Philip C. Habib that his government will meet Wednesday to make "decisions" on the matter.

After meeting for an hour with Habib, who returned here on the latest leg of a two-week Middle East shuttle, Begin effusively praised the U.S. envoy's efforts, and said, in his most optimistic note yet, "Let us express the hope that those efforts will succeed."

While refusing to discuss the content of his talks with Habib, the prime minister said, "Now we shall have to convoke the proper authority to adopt the proper decisions."

He did not specify the "proper authority," nor did he discuss what decisions would be considered. But an informed aide to the prime minister said later tonight that the Israeli Cabinet will meet Wednesday morning, and will consider item by item a U.S-proposed agreement between Syria and Israel designed to defuse the crisis and restore the delicate balance of policy objectives that had existed in Lebanon until the Syrians deployed antiaircraft missiles there in response ot the downing of two Syrian helicopters by Israeli jets.

Begin and Habib are scheduled to meet later in the day to discuss the Cabinet ministers' decisions.

[In Washington, the White House reported that President Reagan met secretly on Saturday with a Saudi Arabian official to discuss the crisis. Deputy Press Secretary Larry Speakes said Reagan and National Security Adviser Richard Allen met at the White House with Prince Turki Faisal, who was described as an adviser to Saudi King Khalid. The Saudis have been working behind the scenes to avert a new armed conflict between Syria and Israel, and Habib had consulted with Saudi officials in Riyadh over the weekend.]

Habib arrived in Jerusalem after two hours of talks in Damascus with Syrian President Hafez Assad. It was his third meeting with the Syrian leader. There were no indications whether he had managed to soften Syria's admamant refusal to remove the missiles, which were installed late last month, Washington Post correspondent Stuart Auerbach reported.

"All I can say is that the diplomatic efforts continue," Habib told reporters in Damascus after meeting with Assad.

Government-controlled newspapers in Damascus, however, continued to press their hard line on the Habib mission, saying that the U.S. envoy should focus not on the missile batteries but on Israeli attacks on Lebanon.

Meanwhile, responding to reports from Damascus that Syrian air defenses had shot down an Israeli reconnaissance aircraft over the Syrian port city of Latakia, and Israeli Army command spokesman in Tel Aviv said, "We completely deny it. No such thing happened."

The Army command spokesman said no aircraft, piloted or otherwise, had been reported missing.

A Syrian military spokesman said that the plane was seen falling into the Mediterranean about six miles southeast of Latakia. It is the third Israeli reconnaissance plane Syria has claimed to have downed in the past week, Auerbach reported.

Tonight, in a speech to Israeli disabled veterans, Begin underscored his newly optimistic tone, declaring, "Israel will never attack Syria unless it is attacked." Previously, he had repeatedly warned that if the Syrian missiles were not removed voluntarily, the Israeli Air Force would bomb them.

Israeli sources close to Begin said that the Habib compromise proposal, while still in a tentative stage, contains these elements:

Syrian peacekeeping forces and Christian militias would disengage in the Sanin Mountains northeast of Beirut.

The Syrians would stop bombarding the Christian city of Zahle, which dominates the strategic Beirut-Damascus road, and regular Lebanese Army troops would police the city.

Israel would agree to stop overflying eastern Lebanon, and particularly the Bekaa Valley, but would get tacit approval to conduct overflights in other parts of Lebanon. Specifically, Israel would not be restricted in its operational flights against Palestinian guerrilla positions in southern Lebanon.

Lebanese President Elias Sarkis would formally request Syria to withdraw its surface-to-air missile batteries from Lebanon, and Syria would begin a staged withdrawal at an unspecified date. Syria would retain its missile batteries inside its border close to lebanon.

Saudi Arabia would resume its financial support of the Syrian peacekeeping force in Lebanon, which was sent there in 1976 under an Arab League mandate.

The United States would attempt to negotiate with Syria and Israel another "red line" agreement -- that is, an understanding about specified zones of operations -- similar to the one tacitly agreed upon in 1976.

The last condition would satisfy Begin's demand for a return to the "status quo ante," which to him represents a restoration of the delicate internal equilibrium that has kept Lebanon in relative balance during the five years the Syrians have been there.

Under the "gentlemen's agreement" of 1976, the Syrians reportedly were prohibited from using aircraft against Christian forces, and from moving south of the Zaharani River, while the Israelis were not to attack Syrian forces. The Israelis were understood to have freedom of movement in Lebanese skies in conducting reconnaissance missions and air strikes against Palestinian guerrilla positions in Lebanon.

Habib tonight refused again to discuss any aspect of his shuttle diplomacy, saying only that he remains hopeful for a peaceful solution.

Begin, in his remarks about Habib, seemed ebullient toward the U.S. envoy, beaming as he said, "I want to express our deep gratitude to our friend, Mr. Philip Habib, for the immense . . . efforts, intellectually, physically, morally, he invests in his efforts to bring about a peaceful solution" in the latest Middle East crisis. "This man is doing his best and his utmost, and we really admire this effort. Let us express hope that those efforts shall succeed," Begin added.