A radically scaled-down design for the billion-dollar PortAmerica riverfront project in Prince George's County was unveiled yesterday by developer James T. Lewis, whose earlier plans for a 52-story skyscraper and then a 42-story tower had been declared hazardous to airplanes.

In place of the controversial World Trade Center he first proposed on the Potomac River site, Lewis said he intends to build an octagonal 22-story hilltop tower with a three-story lobby, and six 10-story office buildings sloping downhill toward the waterfront. The gothic-style complex of buildings would have a campus-like appearance, with a tree-lined mall and an 18-foot waterfall descending into a pool.

Lewis, speaking at a conference in Greenbelt on growth in the 1990s, said his latest design, by architect Philip Johnson, represents the "final form" for the trade center, which he hopes to begin constructing next spring and complete by June 1990.

He said the entire project, consisting of waterfront shops, restaurants, hotels, Georgian-style villas, high-rise apartments and a marina should be finished by the year 2000.

Staff members of the National Capital Planning Commission, which spearheaded opposition to the former designs, said yesterday they are satisfied that there have been "very, very positive changes in the project."

However, Lewis acknowledged a new element of uncertainty in the current fluctuations of the stock market and doubts about the economy. "Since the stock market is troubled, money may be flowing into real estate," he speculated. On the other hand, he said, prospective tenants for such an enterprise are firms that are directly affected by the market.

Lewis said he and his partners have invested $14 million in cash and obtained $20 million in loans for site acquisition and consultants, but the initial construction loan of $250 million is still being negotiated.

The market aside, the worst of Lewis' battles over PortAmerica appear to be behind him. Almost a year ago, Lewis abandoned the skyscraper plan after the Federal Aviation Administration ruled it could interfere with air traffic to and from National Airport. The new design is set at the FAA-approved height limit of 472 feet above sea level.

Since the height issue was resolved, PortAmerica designers have met biweekly with government officials to iron out other issues. Chief among these was the amount of landfill Lewis proposed for his waterfront area. Responding to environmental concerns about the potential loss of aquatic grasses, he scaled down the harbor fill from 35 acres to less than five acres.

"I really have the sense now we're about there," Lewis said. "Most of the issues have been resolved and agreed to."

In the new World Trade Center, pink granite has replaced the reflecting glass of the earlier design. Eight gothic turrets would rise along the sides.

While the design of the office complex, on a part of the PortAmerica site adjoining the Capital Beltway, has changed, Lewis said the total density remains the same, 2 million square feet of commercial space. He said he already has a commitment to lease three underground levels for a World Trade Mart where firms will display wares. It also will include a plaza-type eating area.

Directly behind the World Trade Center, under Lewis' latest design, would be another structure, a seven-floor garage with a 30,000-square foot rooftop health club. Behind that structure, Lewis said he plans to erect three 12-story 150-unit apartment houses, with efficiency units in the $150,000 range.

The new plan must now be submitted to the county planning board, where it is expected to undergo routine review.

Lewis also said he intends to break ground Nov. 24 on a two-story marketing center on Oxon Hill Road at the site. "It will house a one-sixteenth scale model {of PortAmerica} and will probably be a tourist attraction in its own right," he said.