President Reagan, resuming his fierce rhetoric over the downing of a South Korean airliner, said yesterday the Soviet Union has "stonewalled the world" and proved he was right in once saying the Soviets reserve for themselves the right to "commit any crime, to lie, to cheat."

Speaking in his weekly radio broadcast after meeting with Secretary of State George P. Shultz and then with the National Security Council, the president accused the Soviets of a "massive cover-up" of what he again called the "Korean Air Lines massacre," rather than rushing to investigate the crash, apologize and compensate the families of the 269 victims.

At the NSC meeting, Reagan reiterated his preference for maintaining the current size of the U.S. Marine contingent in Lebanon. He was warned, sources said, that there is a possibility that the Marines could be "overwhelmed" and that he would have no choice but to send additional support.

Shultz confirmed Friday that Lebanon has asked the United States and other members of the multinational peace-keeping force in Lebanon to undertake missions in the Chouf Mountains near Beirut as well as other new missions.

However, Shultz said, "Under present circumstances there is not any disposition to change our mission."

On the downing of the South Korean jet, Reagan has been caught between conservative demands for stern retaliation and practical limitations on what he can do. His public remarks have reflected the dilemma.

The president initially denounced the Soviet "lies" and "atrocities," then the next day promised "calm but firm" action. Friday he said "vengeance isn't the name of the game." Yesterday, on the eve of a conservative memorial service for the victims, he castigated the Soviets again.

Reagan said he had been accused of "being too harsh in my language" when in his first press conference as president he said the Soviets had "declared the only morality they recognize is what will further world communism, that they reserve unto themselves the right to commit any crime, to lie, to cheat, in order to attain that.

"Well, I hope the Soviet's recent behavior will dispel any lingering doubts about what kind of regime we are dealing with and what our responsibilities are as trustees of freedom and peace . . . .

"They have stonewalled the world, mobilizing their entire government behind a massive cover-up, then brazenly threatening to kill more men, women and children should another civilian airliner make the same mistake as KAL 007."

Reagan said the Soviets "are terrified of the truth" and have tried unsuccessfully to jam stepped-up Voice of America broadcasts about the airliner crisis. He used the occasion to appeal to Congress for increased funding for the VOA and other international broadcasting programs.

Shultz, who returned Friday from an East-West meeting in Madrid where he protested the airliner incident to Soviet Foreign Minister Andrei A. Gromyko, discussed with Reagan yesterday whether to cancel a tentatively scheduled Sept. 27 meeting between Shultz and Gromyko at the United Nations. No decision was made, according to White House officials.

The issue of the downed jetliner also was taken up by the National Security Council, with consideration given to what Reagan might say in his scheduled Sept. 26 U.N. address to further isolate the Soviet Union.

The council also discussed what the United States might propose at a meeting of the International Civil Aeronautics Organization in Montreal.

On his return flight to Washington Friday, Shultz told reporters that the next likely U.S. response to the Soviet downing of the jet will come at the ICAO.

The western nations are expected to propose rules to improve communications between civil and military airplanes and to ask for rules that would prohibit attacks on civilian airliners in peacetime. State Department sources said the unsatisfactory outcome of Shultz's meeting with Gromyko in Madrid had made across-the-board discussions with Gromyko in New York more difficult.

In his speech in Madrid on Friday, Shultz challenged the Soviet Union "to undertake a serious dialogue" on all outstanding East-West issues. Asked how he can reconcile that statement with his decision to restrict his discussions with Gromyko the day before to the Korean Air Lines disaster and human rights questions, Shultz replied:

"We are ready for genuine discussions, but . . . there has to be somebody on the other side who has that same viewpoint and who is forthcoming and honest about it."

This position as well as Reagan's scathing denuniciation of the Soviets again yesterday suggests a more diffident U.S. attitude toward dialogue with the Soviets than had been the case in this administration.