The Marine Corps MOS 6123 is for Helicopter Power Plants Mechanics, T-64. These mechanics inspect, maintain, test, and repair T-64 engines and power plants. They also perform complete repairs on helicopter power plants and systems.
The Marine Corps also has these MOS codes for helicopter maintenance and mechanics:
6114: Helicopter Mechanic A/UH-1
6116: Tiltrotor Mechanic MV-22
6122: Helicopter P/P Mechanic T-58