By Martyn Herman RIO DE JANEIRO, Aug 15 (Reuters) - Italy's Elia Viviani took the lead after of the track cycling men's omnium as the multi-discipline event headed towards a thrilling conclusion later on Monday. After four of the six disciplines, Viviani was leading with 140 points, 14 more than overnight leader Thomas Boudat of France and Britain's Mark Cavendish. Reigning Olympic champion Lasse Norman Hansen of Denmark, who gained a maximum points haul in Sunday's 60-lap scratch race and 4km individual pursuit but messed up the elimination race, was fourth with world champion Fernando Gaviria fifth. More...