The president of a drilling firm pleaded no contest to involuntary manslaughter in the October, 1985, death of an employee in an elevator shaft hole at a downtown Los Angeles construction site. Under terms of the plea bargain, Michael Maggio, 29, of Maggio Drilling Inc. of West Covina, will be sentenced to a maximum of 90 days in County Jail, Deputy Dist. Atty. Joseph Shidler said. He will also be required to make restitution to the 3-year-old son of the victim, Kevin Bauman, 23, of Baldwin Park, as well as to obtain proper licensing necessary for his job, attend safety classes and to allow state safety inspectors to check his operations. Prosecutors said Maggio failed to monitor air quality in the hole or to shore up its sides, and also neglected to lower Bauman in a safety harness. Burbank Superior Court Judge David A. Workman set sentencing for Dec. 3.