NEW YORK/ MAASTRICHT.- From Manhattan to Maastricht, in the Netherlands, art restorers have spent the last six weeks quietly racing the clock in the annual art world ritual that begins in late January with the old master sales in New York and peaks in Maastricht with the recent opening of the International Fine Art Fair. Each year, the world's top art restorers are called upon by anxious dealers to take their old master paintings for evaluation and restoration. The task must be accomplished quickly —to meet the April 15 deadline — but with exquisite care, so the centuries-old paintings will be ready for the world's most important showcase for old masters. Just how much work a painting will need is unclear until the process is under way. Restoring 17th-century paintings is a tricky business. It always involves expertise, detective work and a keen eye. Mr. Alain Goldrach, who spent years at the Metropolitan Museum of Art and the Museum of Fine Arts, Boston, is regarded as one of the best.