Avco Financial Services has agreed to lease 210,000 square feet of office space at Fluor Corp.'s giant Irvine engineering facility, it was announced Thursday by Fluor, which is seeking to generate new revenue and fill up space emptied by layoffs. The value of the lease was not announced. An additional 100,000 square feet of space in the engineerring building still is vacant and could be leased to other companies, a Fluor spokesman said. Avco Financial, the consumer finance, insurance and credit card subsidiary of Avco Corp. of Greenwich, Conn., recently sold its 16-story Newport Center headquarters building to the Irvine Co. for $45 million. Fluor spokesman Rick Maslin said Thursday that Avco signed a 10-year lease for one-fifth of the useable space in Fluor's four-story engineering compound. Maslin said the lease calls for Avco to move in on Dec. 1.