He fatally shot an NYPD cop execution-style decades ago in a Queens bar — and now Richard Rivera is helping reform police in upstate New York as part of a state-mandated plan launched by Gov. Andrew Cuomo. The cop-killer — who murdered off-duty officer and dad-of-four Robert Walsh in 1981 — sits on a panel for Ithaca and Tompkins County as part of its “Reimagining Public Safety Collaborative.’’ The advisory group was formed after Cuomo ordered municipalities to submit police-reform plans to the state by April 1 following George Floyd’s death.