At this point in the long saga of the skating rink that no one could fix, city officials and the developer Donald J. Trump essentially agree on how to rebuild Central Park's Wollman Rink.

They also agree on how long the job will take, barring further disaster. If the city does it, it will take 18 months. If Mr. Trump does it, it will take four months. Why? The difference, said Alan M. Moss, the head of capital projects for the Parks and Recreation Department, is that the city must adhere to a long list of procedural safeguards - which have only become more prominent since the eruption of the municipal corruption scandal - and that it cannot offer contractors the incentives that Mr. Trump, as a private businessman, can.

Mr. Moss said he believed that ''by keeping a close watch on our schedule,'' the city could open the rink by November 1987. But he saw no reason why Mr. Trump, who in a barbed letter to the Mayor last week offered to take over construction and operation of the rink, could not have it ready a year earlier.

Mr. Trump, not disputing that, makes some additional observations: ''I know how to build. I'm going to take the approach of leadership. I'm going to get good contractors and push the hell out of them. They've worked for me and they want to continue working for me. They want to prove you can do things in this city.