House Democratic leaders and the Reagan administration have agreed on a substitute version of the $8.4 billion appropriation for the International Monetary Fund that they say now has a good chance of passing.

Supporters of the IMF measure hope it can be voted on today.

On Wednesday, House Speaker Thomas P. (Tip) O'Neill (D-Mass.), reflecting a general assessment, had said "the votes just aren't there" to pass the controversial legislation, which would add to the resources of the key international agency dealing with the Third World debt crisis.

But eleventh-hour maneuvering yesterday produced the substitute, which will be offered by House Banking Committee Chairman Fernand J. St Germain (D-R.I.).

For liberals who have bitterly complained that the IMF bill provides a bailout of commercial banks that have become overextended, the amended language contains an "anti-bailout provision" that requires the American executive director of the IMF to oppose any IMF loan designed primarily to repay bank loans "which have been made imprudently."

Conservatives--including gold standard supporters who worry about floating exchange rates--got a new section that would require the president, within six months, to send a report to Congress "regarding proposals to reform the floating exchange rate system."

Any of these provisions may be amended on the floor, and if the bill passes, are subject to change in conference with the Senate.

Although no group of critics appeared totally satisfied, and all sides complained that the St Germain substitute contains more cosmetic than real changes, the consensus last night was that momentum was now moving with the amended biil.

"There's nothing much new in the language," said a moderate Democrat, "but you had to give a lot of people the appearance that they were given something, so as to bring them in."

The substitute commits the United States, through its executive director in the IMF, to "support pro-growth policies" that will lead to "greater financial and exchange rate stability." There is also a bow in the direction of encouraging private sector activity and for "foster ing the creation of democratic institutions."

Increased consultation with Congress on IMF activities is called for by the substitute, including a new semiannual report to both the House and Senate Banking committees by the secretary of the Treasury.

The proposed legislation was still in a state of flux last night. At a late hour, St Germain and Rep. Charles E. Schumer (D-N.Y.) agreed on a modified version of an earlier Schumer amendment that was designed to force banks to charge lower international interest rates to debt-stricken Third World nations. The compromise sets lower rates as a goal, but dropped a specific formula Schumer had proposed earlier.

Treasury Secretary Donald T. Regan praised the compromise, and flatly predicted that a majority of House Republicans would now vote for it, creating the necessary margin for passage. A version of the legislation has already passed the Senate.

St Germain was more cautious on prospects for passage. "I think we are gaining ground," he said.

O'Neill's office would not make a flat judgment on the bill's chances. The speaker was said to be angered because President Reagan had not fought harder for the legislation. The president called in about 20 Republican congressmen Wednseday morning for an arm-twisting session, and appeared to have won at least a few converts.

It is clear nonetheless that that there will still be a strong opposition to any form of the bill. Fred Smith, a spokesman for the Council for a Competitive Economy--a conservative lobby that has opposed the appropriation-- said that the St Germain compromise "is another rhetorical response to substantive comments on the bill." He claimed that opponents remained "firm," but would not predict that the St Germain proposal would lose.

To respond to criticism about the side of the new IMF funding, the substitute makes $2.6 billion of the total conditional on certification by the secretary of Treasury of a serious international financial crisis affecting the entire monetary system.

The $2.6 billion represents a proposed increase in the American participation in the General Agreements to Borrow, a rich-nations' emergency fund. The balance of the $8.4 billion, or $5.8 billion, represents an increase in the United States quota--or deposits--in the IMF, which now total $13 billion.

Essentially, the St Germain substitute only makes explicit the way in which GAB funds were to have been used. But America's European partners, who have placed no strings on their GAB commitments, are likely to be displeased by the substitute's language.

Conservative fears about overextension of American banks may also be assuaged by language designed to limit "excessive lending" by banks, defined as loans that exceed a bank's primary capital.

None of the $8.4 billion provided in the bill--the $2.6 billion for the GAB nor the $5.8 billion in boosted quotas--is actually paid over until needed. Then, it is loaned to the IMF, and the only cost is the net difference between the interest the United States earns, and what it costs the Treasury to raise the money.