A powertrain for a vehicle includes an engine and a method of assembling the powertrain includes providing the engine. A starter mechanism is selectively operable to start the engine. A motor-generator includes an adapter to dispose the motor-generator and the starter mechanism in alternative assembly configurations being a first assembly configuration and a second assembly configuration. The first assembly configuration is when the starter mechanism is coupleable to the adapter to selectively transfer torque from the motor-generator through the adapter and the starter mechanism to start the engine. The second assembly configuration is when the starter mechanism is spaced from the adapter to operate independently of the motor-generator such that the starter mechanism selectively transfers torque to start the engine.