Madison Square Garden Corp., the corporate owner of the Washington Diplomats, is likely to begin seeking a buyer for the team before the end of this month unless the North American Soccer League votes next week to drastically alter its makeup and scheduling for the 1981 season, according to informed sources.

If a buyer cannot be found -- the Minnesota franchise has been on the market and unsold since June -- then Garden management might decide to turn its certificate of ownership over to the league and leave the franchise in the hands of NASL Commissioner Phil Woosnam. That possibility already has been discussed by club officials, sources said.

"The last thing in the world we want to see is Madison Square Garden leave the league." Woosnam said yesterday. "I think the proposals they favor in terms of scheduling and playing natural rivals more often have a decent chance of getting through."

"If they did decide to sell, then I would still want to see to it that Washington would still have a franchise. I would immediately begin working to try and find a buyer who would keep the club in Washington next year. I think the Washington franchise made more progress last year than any franchise ever in the league. I wouldn't want to see it moved."

Woosnam said that if he thought his intervention during the league meetings could bring about a compromise that would keep Werblin in the NASL, he would do it immediately. "I want to see Madison Square Garden in the league," he said. "I think they've done a great job."

MSG Chairman Sonny Werblin and Executive Vice President Jack Krumpe have been openly critical of the NASL almost since the day the Garden purchased the Diplomats in October 1978.

Both men have said on more than one occasion that they think the current scheduling setup is ludicrous, with teams constantly traveling coast to coast and playing natural geographic rivals only twice in a season.

League sources say that Werblin and Krumpe will try to push through a proposal that would divide the league into three eight-team conferences, with teams playing only opponents in their own conference, in the regular season.

Under that proposal, Washington would be in a conference with the Cosmos -- the league's glamour team -- Toronto, Rochester, Montreal, Tampa Bay, Fort Lauderdale and New England, all Eastern teams.

League sources say that, as of the moment, the Werblin-Krumpe proposals would not pass.

Werblin and Krumpe would not comment on the situation this week, insisting that they would not make a decision on the Diplomats until after the league meetings Monday, Tuesday and Wednesday in Toronto.

But sources say that a decision already has been made: if the league is not willing to make what Garden officials believe are crucial changes in its basic structure, then it will be time for MSG to get out of the league.

"No one has any problems with the Washington market," one source said. "THE PROBLEM IS WITH THE NASL."

Madison Square Garden has had losses totaling close to $5 million in the two years it has owned the team. No club in the NASL will make money in 1980 because expenses cannot possibly overcome the huge costs of acquiring and paying foreign players.

Garden officials also are very concerned about the league's unsettled labor situation. The league has been ordered to negotiate a collective-bargaining contract with the NASL Players Association.

"They're handling the labor situation like everything else," one club official said of the league. "Like they expect a bolt of lightning to strike and make everything okay."

The Diplomats' chances of getting their proposals through the league meeting successfully are not considered good. Although some of the other more powerful and wealthy clubs in the league would probably go along with the proposals, many other clubs resent teams with corporate ownership and are inclined to vote against anything they propose.

It also is unlikely that the 16 teams that would not be in the Cosmos-Washington-Fort Lauderdale-Tampa Bay conference would vote in favor of the move. They would be giving up home dates with four of the league's five glamour teams. Only Seattle, among the league's best teams, would not be in that conference, and the Cosmos carry more prestige than the rest of the league combined.

Thus, barring some very successful arm-twisting by MSG management in Toronto, or a compromise, the Diplomats are likely to be sold. Team President Steve Danzansky has first-refusal rights if the club is put on the market. "I don't want to comment on something that may never happen," he said.

One clue to the situation may have been the seemingly innocent announcement Tuesday that Werblin's son, Thomas, is leaving the Dips to become a salesman for Madison Square Garden Communications. Werblin's work with the Dips this past season was generally held in high regard. His leaving now would indicate that the soccer team is not the priority it was a year ago when Werblin came here.

Also significant has been the sudden reluctance of Werblin and Krumpe to discuss their criticims of the league. Apparently, neither man wants to appear as if he is delivering an ultimatium to league owners at this point.

"Don't bet on Sonny Werblin for 1981," said one source. "He's had it with this league. If they aren't ready to change, he'll get out."