Major safety modifications on aging Titan II missiles may be two years away, an Air Force spokesman said yesterday in announcing that $12 million has been added to the service's 1981-82 budgets to design the changes.

About 100 modifications in the 18-year-old Titan II ICBM system were recommended by an Air Force team that looked into operation of the liquid-fueled missiles after September's explosion at a site near Damascus, Ark., that killed one airman and injured 21 others.

Disclosure of the Air Force plan was made as spokesmen for the Strategic Air Command confirmed that a minor oxidizer leak had occurred Thursday afternoon at a Titan II site near Searcy, Ark., about 45 miles east of Damascus. No one was injured as the leak delayed a normal refueling operation for five hours.

Congressional sources voiced concern yesterday about the length of time between designing Titan modifications and putting them into operation.

Air Force officers cited as one example the need to spend $700,000 this year to design a siren warning system for the 52 Titan II sites around the country to warn persons living or working nearby of a hazard.

"It will be 1982 before we can put that system at sites," one officer said yesterday.

Replacement of current vapor detection systems will not be funded until next year, according to Air Force officials.

The fixed and portable vapor detecting systems, which provide the main warning for airmen that deadly fuel or oxidozer may be leaking, no longer work properly.

The fixed system, the latest Air Force safety study showed, was not operating 40 percent of the time. The portable system, which under new orders must be carried by all airmen inside a missile silo, was also found unreliable.

Another major safety problem has been the safety suit an airman must wear when handling fuel and oxidizers.

Under current plans, $2 million is to be provided in next year's budget to buy new suits. Meanwhile, airmen must wear suits that, the study found, are so limited in number and size that refueling teams are sometimes formed "to fit the suits instead of the people to fit the tasks."

A newly designed safety suit was recommended after an accident in 1978 in Rock, Kan., that killed two airmen. Now, according to Air Force sources, that new suit may not be available until 1983.

As part of this year's added spending, the Air Force is making simple changes at each site such as acquiring new rubber mats that fit between the missile and work platforms. The wrench socket that fell and punctured the missile fuel tank at Damascus dropped through a rubber mat.