DETROIT (AP) _ Florida publisher Frank Vega will replace Joseph Ungaro as president of the financially struggling Detroit Newspaper Agency, a newspaper reported Wednesday.

The agency's five-member board, comprising representatives of the parent companies of The Detroit News and Detroit Free Press, is to announce the move Thursday when it meets in Detroit, sources told the Free Press.

The DNA was formed 14 months ago to administer the newspapers' joint advertising, circulation and production operations. The News and Free Press maintain separate newsrooms.

The joint operating agreement was meant to halt years of mounting losses by the two Detroit papers and the possible failure of the 160-year-old Free Press.

The DNA reported 1990 advertising and circulation revenues totaling $391.6 million. It lost about $12 million, roughly half of what was lost in 1989. DNA officials said earlier this year that they hoped to limit 1991 losses to $6 million, then break even in 1992.

Sheila Gibbons, spokeswoman for Gannett Co. Inc., owner of the News, said Wednesday that Gannett couldn't confirm the report on Vega and Ungaro.

Officials at Knight-Ridder Inc., owner of The Free Press, didn't return calls for comment Wednesday.

A spokeswoman in Ungaro's office said he wouldn't comment. Vega didn't return calls on Wednesday.

Under the joint operating agreement, 450 jobs were cut at the News and Free Press and the News ended morning home delivery, which has cost it more than 194,000 daily readers since December 1989.

Daily circulation of the News and Free Press, and that of their combined weekend editions, has continued slipping amid an ailing Michigan economy and lingering problems in coordinating delivery and circulation.

''A lot depends on what happens with the economy,'' said Robert Kurzawa, senior vice president of circulation for the DNA. ''If things pick up, people might feel better about taking two papers.''

Vega, publisher of Gannett-owned Florida Today in Melbourne, Fla., visited Detroit earlier this year to study DNA circulation operations. He was the first circulation director of USA Today, Gannett's nationally circulated daily.

Ungaro is a former publisher of Gannett's Westchester-Rockland (N.Y.) papers.