A federal grand jury indicated former Rep. Richard T. Hanna (D-Calif.) yesterday as a central participant in South Korean influence-buying on Capitol Hill, charging that while in Congress he traveled to Seoul in 1967 and helped Korean intelligence agents plot their campaign.

The grand jury said Hanna acted illegally as an agent of the Korean government while in the House, suggesting which members of Congress Korean agents might most profitably approach.

Hanna, the first present or former member of Congress to be formally charged in the Korean case, was indicted on 40 felony counts involving bribery, conspiracy, mail fraud and failure to register as a foreign agent.

Justice Department officials said they could recall no previous case in which a congressman was charged as an illegal agent of a foreign country.

The indictment indicates that Hanna, at a 1967 meeting in Seoul with the director of Korea's Central Intelligence Agency, helped hatch the original plot "corruptly to influence U.S. congressmen to take actions favorable to the Republic of Korea."

For the next seven years, according to the indictment, Hanna was an active participant, targeting colleagues in Congress for Korean agents to approach and working himself to encourage other congressment to support the Seoul government.

Hanna regularly wrote South Korean President Park Chung Hee and other top Seoul officials with optimistic reports on the progress of the scheme, the indictment says.

In one letter Hanna, a liberal Democrat, wrote to Seoul, soon after Gerald Ford became President, that Korea "could expect better relations with the executive branch . . . because of Tongsun Park's friendship with persons friendly to President Ford."

The indictment portrays symbiotic relationship between Hanna and Tonsun Park, the Korean businessman and socialite who was indicted her in August for his role as a secret agent in the influence-buying scheme.

Hanna used his position as a congressman, the indictment says, to help Park gain a position as the agent for millions of dollars of Korean rice purchases in the United States. Park used his rice commissions, in turn, to make contributions and cahs gifts to members of Congress - including Hanna - the grand jury charged.

Hanna was named an unindicted co-conspirator when Park was indicted, and Park yesterday was named an unindicted co-conspirator in the Hanna indictment. Two former directors of the Korean CIA were also named Hanna's unindicted co-conspirators, as they had earlier been named Park's.

In most cases, co-conspirators would be indicted in a single indictment. But there was apparently some haste last summer to indic Park - who had refused to return from Seoul to face the charges - and the Justice Department indicted when it did not at that time have sufficient evidence to indict Hanna as well.

Hanna could be tried even if Park does not return here, but a trial is probably months away, according to Thomas Henderson, chief of Justice's Public Integrity Section.

Henderson predicted numerous pretrial legal arguments, including several battles as to whether Hanna is immune from prosecution under the Constitution's "speech and debate clause, which protests members of Congress from criminal liability for official activities.

Hannah, 63, who now lives in Fay-etteville, Ark, was not available for comment. His Washington attorneys, Charles A.McNeils, said simply, "We shall meet the government in court."

The most serious charges against the former congressmen are the bribery counts, alleging that he received "an amount exceeding $100,000 and other other things of value" from Tongsun Park between 967 and 976.

The indictment includes one count under the Foreign Agents Registration Act, which requires that anyone working in the U.S. as an "agent of a foreign principal" must register as such with the Justice Department.

The grand jury charges that Hanna acted as an agent of Tongsun Park and two successive directors of the KCIA to "indoctrinate, convert, induce, persuade and in other ways influence" members of Congress and other U.S. officials. The indictment says Hanna never registered as a foreign agent.

The charges set forth various steps by Hanna and Park to further their plan to corruptly influence members of Congress, including the founding of the George Town Club, where Park entertained members of Congress in luxurious surroundings, and Park's payments and contributions to several members of Congress.

Most of this information is drawn verbatim from the Park indictment returned in August. The only congressman named yesterday who was not cited in Park's indictment was Edward J. Patten (D-N.J.)

The indictment said that Patten, at the urging of Hanna and Tongsun Park, sent Park Chung Hee a letter in 1971 "reporting the efforts of Tongsun Park . . ." It said Patten also submitted to a House subcommittee, in modified form, a statement ghostwritten by Hanna that praised the South Korean government.

Patten's office said yesterday afternoon that Patten was en route home and could not be reached. He has been a strong supporter of South Korea throughout his 14 years in Congress.

The indictment also includes 35 counts of mail fraud. These charges involved checks mailed from Tongsun Park to Hanna, and several letters Hanna sent to Korean officials reporting on the influence-buying scheme.