Certron Corp. reported net income of $492,000 for the fiscal year ended Oct. 31, a 48% increase from the $333,000 posted last year. The fiscal 1987 figure includes a $120,000 tax credit, and the 1986 results reflect a pretax charge of $123,000 related to the closing of the company's injection molding facility. Net sales for fiscal 1987 were $27.8 million, up 7% from the $25.9 million reported a year ago. The company said fourth-quarter results were not available. Certron Corp., Anaheim, manufactures and distributes audio-recording tapes and floppy disks, distributes videocassettes to the consumer market and, in its Mexicali facility, manufactures and assembles products for other companies.