BALBOA, PANAMA — SHIPPING traffic diverted by the Gulf war gave the Panama Canal a record year. For the fiscal year ending Sept. 30, some 193 million tons of cargo passed through the canal (up 5.7 percent over 1990), generating $374.6 million in toll revenue.But 1991 was an anomaly. Since the mid-1980s, canal transits and revenue have been relatively flat. Future growth looks modest, at best. "The key countries using the canal, Japan and the US, have large, mature economies," says Richard Wainio, director of executive planning for the Panama Canal Commission. "That implies slow growth in trade on routes between these countries. The most rapid world trade growth is along inter-Asian routes." The canal operations have never been a big money-spinner. But the related port and shipping services do make a valuable contribution to the nation's economy. And there are concerns about Panama's ability to take over control of the US-operated canal in less than nine years, as mandated by a 1977 treaty. "So far the track record has not been good. Look at the shape of the ports, the railroad, and Panama's debt," notes an American canal commission official. President Guillermo Endara's government has just begun (none too soon, say shippers) to address the challenge. On Sept. 26, an "ad hoc" presidential commission released a preliminary report on administering the canal and the surrounding 364,078 acres of land that are now partially under Panama's control. The commission recommends that by next April - after consulting politicians, businessmen, and public interest groups - a legislatively approved entity should be established to deal with development proje cts on former canal lands. And an independent canal administration agency should be set up by late 1993. "The new canal entity should be as immune from politics as possible. We don't want to undermine canal efficiency," says Adolfo De Obarrio, a commission member. The board of directors, he says, should be elected at long enough intervals to prevent Bany elected government from controlling the canal a[Bdministration. Mr. Wainio says Panama must convince shippers as soon as possible that it is well-prepared to take over the canal operations. "Many companies are already looking at contingency plans."