Twenty thousand Japanese fight fans abandoned their traditional Oriental reserve last week at Tokyo's Korakuen Stadium to cheer in patriotic frenzy as their challenger, Kenji Yonekura, took on Bantamweight Champion Jos� Becerra of Mexico. But Becerra showed up with enthusiastic backers of his own: 150 swarthy aficionados who came 7,000 miles to behave at ringside like heated Mexican jumping beans. "Viva Mexico!" they shouted when Becerra won a split decision. An international answer came from the disappointed 20,000: "Boo!"