There's a few problems with the way that you have presented this question and scenario.

At the time, there was nothing illegal or dangerous proven to be occuring at Jonestown. Multiple family members of cultists expressed concern to Ryan, who himself was friends with family members of the Cult. Many of his constituents were made up of members of the Cult and their family members as he was representative of California's 11th District, which is part of the San Francisco/Oakland metro area, the place where the Cult was based out of.

Additionally, just as a correction, Ryan was a member of the House of Representatives, not the Senate. Two different houses with different rules, regulations, electoral policies, and responsibilities (clarifying in case you aren't American.) This is a page from Congresses' official website, which can be tricky, and this is the wiki article for simplicities sake. It's a difference that does matter. Not trying to be snotty, just wanting to help.

After Ryan and a team from NBC news had visited the compound, they were murdered in an ambush at the airport on November 18th, 1978. At the same time Jim Jones, the cult leader began the mass suicide act so there was no time to react to the cults murder of Ryan and his investigation.

Mind you Ryans investigation was taken under his own purview as representative of the 11th District. He was acting in no official capacity as a member of the U.S. Government. He had no ability to speak in a diplomatic manner, no ability to enforce American laws, or to initiate military action. This is because of the Logan Act. A law dating all the way back to 1799. While at the same time, quite often Congress does participate in acts that to play around with this wonky law and the separation of powers acting as semi-official diplomats. These are often referred to as Congressional Delegations, or CoDels and are undertaken by appropriate requests and correspondence with the relevant Congressional Committees. Ryan did form an official CoDel to investigate the facility and Cult.

As for what would have occurred if Ryan had not been murdered or the mass suicide happened is purely speculative. However, if enough cause for concern was found, the U.S. State Department would probably have put pressure on the Guianese government to arrest and extradite cult members.

Military intervention almost certainly would have been off the table. Carter's administration conducted very few military operations under his ability to utilize them in emergency situations. The largest use of the American military in his entire Presidency was the failed hostage rescue attempt in Iran. Carter's foreign policy in Latin America was very focused on internal sovereignty and democracy with elements of diplomatic co-operation according to this RAND study in 1980. His administration according to this Carter Center was more focused on utilizing diplomacy over military pressure in his Latin American actions, and was focused more on Nicaragua, Mexico, and Cuba. Most likely Carter would not have violated Guianese sovreignty by utilizing military force to capture Jonestown and return him to the U.S. for trial. The closest event along these lines would have been the U.S. invasion of Panama in 1989.