THE International Monetary Fund is helping the republics of the former Soviet Union set themselves up financially as separate nations.IMF staff is providing extensive policy advice and technical assistance in setting up central banks, a commercial banking structure, a statistics-gathering organization, and a national accounts system. A group of 120 to 130 experts is also consulting in regard to fiscal policy and administration and other "macroeconomic" (big picture) policies. This, notes an IMF spokesman, is being done in the framework of the "special association" agreement with the Soviet Union signed by former President Mikhail Gorbachev and IMF managing director Michel Camdessus on Oct. 5 in Moscow. That agreement provided for assistance to the republics if requested. And though the Soviet Union itself has gone out of business, the help continues under that agreement. "It's a weird situation," concedes the spokesman. With the exception of Russia, the IMF will be starting almost from scratch in the republics in establishing the financial apparatus of sovereign states. By the new year, IMF officials will have visited all 12 Soviet republics. In the first month or two of the new year, the IMF hopes to complete agreements with eight of the republics on programs for stabilization and transition to a market economy. These eight signed an agreement on the external debts of the ex-Soviet Union with the Group of Seven industrial democracies in November. Three former Soviet republics, Latvia, Lithuania, and Estonia, have already applied for membership in the IMF and its sister institution, the World Bank. They are expected to be admitted by mid-1992. However, the question of membership for Russia, the Ukraine, Kazakhstan, and the other republics could be a tougher issue. The three Baltic states are tiny and would need only small quotas - and thereby limited voting rights - in the IMF. But Russia, especially, is a major nation and would expect a sizable quota. In admitting Russia, the major Western industrial nations will be reluctant to lose too much voting clout within the IMF. The US in particular would like to keep its current veto on important is sues before the IMF. The Fund has set up a separate group (Europe 2) under John Odling-Smee, a British citizen, to deal with these former communist states. World Bank President Lewis Preston signed a technical cooperation agreement with Mr. Gorbachev, then Soviet president, on Nov. 5. That deal continues with individual republics. Using a $30 million "technical assistance trust fund" established in September, the bank is giving priority to reform of the food sector. "Widespread reforms are called for throughout the food chain and especially to increase production and to improve storage, processing, transportation, and distribution," Mr. Preston said on signing the deal. "Losses can be reduced, both on and off the farm." The bank's experts will be working with staff from the IMF, the European Bank for Reconstruction and Development, and the European Community. The goal is a market-based economy.