President Reagan is prepared to announce a revised U.S. negotiating position at the strategic nuclear arms reduction talks (START) in Geneva that would call for using warheads rather than missiles as the measure of the U.S. and Soviet long-range nuclear arsenals, administration officials said yesterday.

The change was a principal recommendation of the President's Commission on Strategic Forces, better known as the Scowcroft Commission, which also called for U.S. deployment of the MX intercontinental ballistic missile and "vigorous pursuit" of negotiated arms control.

Questioned by reporters at a Rose Garden ceremony yesterday, Reagan said he would be "very flexible" in his new approach to the Soviets in the START talks.

He also chatted at another ceremony with Senate Foreign Relations Committee Chairman Charles H. Percy (R-Ill.), who afterward called for an urgent summit meeting between Reagan and Soviet leader Yuri V. Andropov to discuss the nuclear arms race.

"Time is running out," Percy said.

Reagan answered that "We never ruled that out, but I think there has to be something to discuss, though." He and his aides have often said that a U.S.-Soviet summit must have a pre-arranged agenda and a reasonable chance of success.

Administration officials said the president is "in the process of deciding" a number of unresolved issues in revising the U.S. START proposal. These include whether there should be a limitation on the lifting power, or throw weight, of intercontinental nuclear missiles and how to frame a "build-down" proposal in which both sides would destroy two nuclear weapons for each new one they deploy.

"Build-down ultimately will be included in some form, but there are difficult and technical problems as to how to incorporate it into a negotiating position," a knowledgeable U.S. official said yesterday. "We're not there yet."

Reagan is expected to reach some general decisions on the revised U.S. negotiating position at a meeting of the National Security Council today, leaving a number of the technical questions to be worked out later.

Immediately after the NSC meeting, Reagan is scheduled to meet with key members of Congress of both parties who have made presidential support of nuclear arms reduction a condition of support for deployment of the MX intercontinental ballistic missile. He is to discuss his position again in a bipartisan leadership meeting on Wednesday morning and make an announcement of the new U.S. position the same day, officials said.

House support for the MX remains shaky, as was reflected in a letter sent late last week to Reagan by 14 moderate congressmen, 10 of them Democrats, who backed the MX in test votes last month.

The letter, drafted by Reps. Albert Gore Jr. (D-Tenn.) and Norman D. Dicks (D-Wash.), informed Reagan that the congressmen "have no doubt about your sincerity or your commitment" to arms control but said "we are concerned that the message has not reached" key members of the administration.

In an unmistakable reference to Secretary of Defense Caspar W. Weinberger, the congressmen argued that it would be wrong "to base our new position on an unrealistic demand for equal throw-weight."

Weinberger has argued that throw weight must be considered in any revised U.S. negotiating proposal. The congressmen's letter quoted testimony of Paul H. Nitze, the chief U.S. negotiator at Geneva, that the Soviet advantage in throw weight "is so great that it is hard to figure an agreement that would deal with throw weight and have a chance of being negotiable."

Reagan is to meet today with the congressmen who wrote the letter in an effort to ressure them once again of his commitment to obtaining a strategic nuclear arms agreement.

Meanwhile, dissatisfaction among House Democrats who oppose the MX over the president's recent victory on the issue came to a head as 109 Democrats signed a petition for a party caucus on it a week from today. The petition drive was organized by a little-known freshman, Rep. Jim Bates (D-Calif.), who said he became "frustrated and disappointed" after 91 Democrats voted for the MX.

"The Democratic Party platform opposed the MX," Bates said. "Many freshmen campaigned on their opposition to the MX. Yet every member of the leadership voted for it, with the exception of the speaker, who did not vote."

Rep. Les AuCoin (D-Ore.), an outspoken MX foe, said a caucus is needed "because there's a lot of bad blood--a lot of members are bitter. They feel sandbagged by the leadership and the moderate Democrats who supported the administration."

But it is these moderate Democrats who most concern the administration. One administration official observed that Reagan had made a commitment to moderates of both parties to include build-down in the U.S. proposal that will be submitted in the next round of START talks but said this did not mean that the entire proposal would be ready when the talks resume Wednesday.

As administration officials described the process yesterday, the president and his national security advisers are concerned that the confidentiality of the arms negotiations be maintained.

This means, they said, that Reagan will announce U.S. positions only in general terms in an effort to preserve "negotiating room" and discourage the Soviets from making a public statement rejecting the new U.S. position.

"Maintaining the integrity of the negotiating process requires a delicate, diplomatic effort," an official said.

But Reagan also is engaged in a political effort to reassure congressional moderates and U.S. allies abroad that he is committed to serious arms control negotiations with the Soviets.

One official said that "there are those in Europe" who will use the new U.S. stance in the START talks as a clue to U.S. intentions in the separate negotiations on reducing intermediate-range nuclear weapons (INF) which resumed last month in Geneva. There are those in the administration who think that the two negotiations ultimately may be merged.