Donald Duck paddles ashore soon on Tokyo Bay

It's quite possible Cinderella would have had second thoughts about marrying her handsome Prince Charming if his castle had been located here.

Yet here it is - glittering spires soaring up from drab, wind-swept reclaimed land on the upper reaches of Tokyo Bay, a somewhat incongruous sight among a jumble of identical high-rise, low-cost apartment blocks, prefabricated houses, and factories.

But Urayasu (it means ''shallow creek'') - formerly an exit sign on the highway to the new Tokyo International Airport at Narita - is undergoing a fairy tale transformation. It will become the Magic Kingdom, the world's first Disney amusement park outside the US, to be opened next April 15.

And perhaps the most magical touch: It isn't costing the Disney organization a cent. For years the American entertainment giant resisted offers to build a Disney theme park overseas. An early study of the Japanese market, which found both great potential and probable major problems, got nowhere when Disney concentrated instead on building Disney World in Florida, which opened in 1971.

But in the end, the Japanese made Disney an offer even old Scrooge McDuck could not have resisted.

Oriental Land, owned primarily by real estate and private railway interests, offered to build and operate the park. All it wanted was the Disney name, and its accumulated expertise, in return for royalties (10 percent on admissions and rides and 5 percent on merchandise, food, and beverages).

Stock analysts expect the American firm to make at least $14 million to $15 million in the first year. Disney has already benefitted from selling Oriental Land some $100 million worth of rides and technology.

In most respects Tokyo Disneyland is a clone of the well-established operations in California and Florida, but Frank P. Stanek, vice-president of Walt Disney Productions Japan, says the Japanese operation is also an improved version reflecting the current state of Disney technical magic.

''We've got the best of both the (US) parks here,'' he says, while Jack Myers , director of marketing, insists: ''The shows are going to be better here. . . . We're going to correct our past mistakes.''

Oriental Land officials are convinced they have a winner. They reckon on at least 30 million potential fans of Mickey Mouse, Snow White, et al, living within a 30 mile radius of Urayasu (only 20 minutes by train from central Tokyo).

There is no doubt Disney is popular. Disneyland is regarded as a virtual must for Japanese visitors to the US, while Disney movie masterpieces show no signs of losing their timeless appeal. Japanese-dubbed versions of ''Cinderella'' and ''Lady and the Tramp,'' for example, have been drawing capacity audiences in the Tokyo area ever since the past summer's school vacation (this correspondent had to take his young daughter three times).

But it has taken more than the mere touch of a magic wand to get Tokyo Disneyland off the ground. There have been considerable cost overruns, with major banks being repeatedly asked to play the role of a fairy godmother. The latest estimate is that the Magic Kingdom will cost some $600 million to be completed, with an estimated three-quarters of the cost covered by bank loans.

The Urayasu project will have to operate under some contraints not applicable to the California and Florida operations - such as much colder winters and a wet , typhoon-laden summer to discourage visitors. And it won't be fun on the cheap. Oriental Land is figuring on families spending an average of $80 to $90 per visit.

But both Disney and Oriental Land are convinced Mickey Mouse and his friends will exercise the same irresistible lure as they have already to some 300 million visitors to the US facilities.

From there, visitors will move along landscaped walkways to the five theme areas: World Bazaar, Adventureland, Westernland, Fantasyland, and Tomorrowland. As a concession to the unpredictable Tokyo weather, most of the facilities will be roofed over.

Construction is now almost 90 percent complete, leaving its promoters with little else to do but wait for April 15 and ''wish upon a star.''