After long months of negotiation, the Reagan administration has agreed to a Senate plan to revitalize Union Station. The plan would complete roof repairs already under way and finish the parking garage begun several years ago. If feasible, structural repairs would be made and contracts signed with private firms to develop a commercial complex in and around the monumental Beaux Arts-style station.

A Senate committee is scheduled to mark up a bill today outlining the plan, which also would move control of Union Station to the Transportation Department from Interior. Yesterday, during a lunch at The Washington Post, Transportation Secretary Drew Lewis indicated that he supports the bill's basic elements.

The new plan is the latest in a series of costly and unsuccessful federal efforts to rescue the 74-year-old station, once among the world's grandest rail gateways but today a crumbling white elephant with corroded plumbing and a leaky roof.

On July 4, 1976, the Interior Department opened Union Station as a National Visitor Center, moving trains to a smaller, modern structure to the rear. Despite $46 million spent on the conversion, the station never caught on in this role and it continued to deteriorate structurally. In February it was closed after rainwater seeped through the roof, dislodging huge chunks of plaster from the ceilng and flooding carpeted floors.

Committee sources predicted yesterday that the bill will be approved by the Senate Environment and Public Works Committee and has a good chance to make it through the full Senate. After that it would go to the House, where the outcome appears less certain. There is no equivalent legislation in the House.

As now worded, the bill calls for Interior to complete $8 million of already funded roof repairs and to turn the building's management over to the Transportation Department. The bill also authorizes the District of Columbia to trade in $40 million in federal highway funds for a like sum to complete a large parking garage last worked on in 1976.

The garage money represented a concession on Lewis' part. In testimony before Congress last July, he opposed allowing the city to shift its funding that way.

In addition, the Senate bill would authorize $1 million for two major studies of Union Station. One would survey the building's structural problems and determine what repairs are needed and how much they would cost. The other would explore the feasibility of entering into joint development of the station and surrounding property with private developers.

Congressional concern over the station was revived last February following the station's closure. The bill now under consideration is being shepherded through Environment and Public Works by Sen. Slade Gorton (R-Wash.).

Earlier versions of the bill proposed that large sums be diverted to Union Station from ongoing capital programs and to modernize rail lines along the Northeastern corridor. Lewis strongly opposed such diversions.

Both sides, however, agreed that action was needed to save a national monument like Union Station. In negotiations between Senate and Transportation Department staff members, alternative wording was worked out that would take only $1 million from the Northeast Corridor funding to pay for the two studies. The department would be free to finance certain other spending as it chose.

The development study would explore running transportation to area airports from the station, and constructing a helicopter pad. Lewis would have to report back to Congress within a year with a specific plan for revitalization, assuming that it was found to be economically feasible.

The bill would authorize the department to buy the station over a six-year period (it currently is leased from private railroad interests). The department also could make further repairs to the building, buy surrounding property and contract with private companies for development, under the plan.

The bill specifies that the old building must again become a rail station, with ticketing, baggage handling and other such functions. However, only a fraction of the mammoth station would be needed for this, with remaining space occupied by retail stores and restaurants.

Senate sources said that though the bill contains no specific requirement, its intent is to remove the new station behind the old one where rail passengers now board their trains.