A federal judge said yesterday that U.S. Customs Service agents may have acted improperly last year when they seized evidence and arrested three businessmen suspected of trying to sell a diesel engine assembly plant to the Soviet Union.

District Judge Oren R. Lewis told prosecutors Joseph Aronica and Dennis Szybala they have until Tuesday morning to explain why the three defendants were arrested Dec. 28 without warrants.

Lewis said during a hearing in U. S. District Court in Alexandria he was "utterly amazed" that customs agents arrested the three and seized their briefcases without court-approved arrest or search warrants.

"I'll give you until Tuesday morning to give me some authority other than 'Aronica on Arrests'," Lewis told the government lawyers.

The defendants--Paul Sakwa, a former CIA agent who lives in Northwest Washington, Stephen G. Carter of Chicago and Gerald F. McCall of Toronto--are charged with violating U.S. conspiracy and export laws by attempting to sell the engine assembly equipment without an export license.

All three were indicted by a federal grand jury in Alexandria early last month.

Customs agent John Meyers, who led a three-month investigation of the men while acting undercover, told Lewis he contacted Aronica on the morning of the arrests and was told that a warrant was unnecessary. Carter and McCall were arrested at National Airport. Sakwa was taken into custody at a Crystal City hotel.

The government maintained to the skeptical Lewis it had "probable cause" to arrest all three without warrants.

Defense lawyers also criticized the agents for seizing documents without search warrants. Briefcases belonging to all three were taken by the agents, but were not searched until five hours later, according to testimony.

"I don't think that's even close to a search incident to an arrest," said Lewis.