Raytheon is pitching a new upgrade for the venerable M60A3 Patton main battle tank that would turn the elderly design into a competitive force on the modern battlefield—all at a fraction of the cost of a new vehicle. Named the Raytheon M60A3 Service Life Extension Program (SLEP), the upgrade is being offered for export to nations that need the performance to take on threats like a Russian-built T-90S, but can’t afford a top-of-the-line machine like the M1A2 SEP(v)3 Abrams or Leopard 2A7. At the core of the upgrade is a new 950-horsepower diesel engine—which replaces the original 750-horsepower unit. As...