The American defense establishment needs to be overhauled to take account of a lessened threat from the Soviet Union and heightened risks in other areas, says a study by a private research organization.

Soviet leader Mikhail Gorbachev's internal reform efforts have somewhat lessened the military threat to the United States at the same time that threats from other sources, such as terrorism and economic-based problems, have increased, concluded the National Security Group report."Due to Mr. Gorbachev's initiatives, that threat appears to have decreased. At the same time, we find ourselves increasingly threatened by smaller, Third World countries, terrorism and unconventional problems like drugs, environmental pollution and instability," the study said.

"In the face of this horizontal escalation, we find that our force structure, intelligence system and operating procedures are all geared to the large single threat; we do not have the flexibility required for these smaller, more diffuse hazards," it said.

The National Security Group describes itself as a non-partisan educational organization. The five-year-old organization is based in Chicago and has previously sponsored studies on security-related issues such as arms control. It board of advisers includes Donald Rumsfeld, the former defense secretary.

The report said "a new ominous potential threat looms in the growing influence of the drug cartels of Latin America over the legitimate governments of that area."

The solutions it proposed include a "Marshall Plan" for Latin America, increased flexibility in the U.S. military structure and intelligence-gathering apparatus, and new attention to global environmental problems.

The report also urged the winner of next Tuesday's presidential election, either Democrat Michael Dukakis or Republican George Bush, to quickly call a "budget summit" with Capitol Hill leaders to deal with looming defense budget problems.

The Pentagon budget will be a key concern of the new administration. Defense spending rose sharply each year during President Reagan's first term but has been essentially frozen during his second term. The current defense budget is $300 billion and is not likely to rise much regardless of who is elected Nov. 8.

"Overly optimistic projections of defense budget growth coupled with relatively larger growth of internal fixed accounts and public pressure against increasing defense spending will result in a decrease of $250 billion to $400 billion for defense in the next five years from what the military had assumed," it said.

During Reagan's first term, the military services made down payments on a large number of new weapons systems. Some of those weapons may have to be canceled.

The report also said the Pentagon spending effort should be broadened to keep pace with the new view.