Plucky kids from Harlem fiddled their way into the hearts of Jeff Nelson, whose relief pitching helped the Yankees win four World Series, and Marcellus Wiley, the former Buffalo Bill now on ESPN, at a fund-raiser for the cash-strapped Opus 118 music school. The two men were blown away at El Museo del Barrio Thursday night by the virtuoso skills of Harlem kids who get classes for free through the program made famous in “Music of the Heart” (1999), which starred Meryl Streep as Opus 118 founder Roberta Guaspari.