A powertrain for a vehicle and a method of assembling the powertrain are disclosed. An engine includes a crankshaft being rotatable about a longitudinal axis. A ring gear is attached to a first distal end of the crankshaft such that the ring gear and the crankshaft are rotatable in unison about the longitudinal axis. A starter mechanism is selectively operable to rotate the ring gear and the crankshaft to start the engine. A motor-generator includes an adapter selectively cooperating with the starter mechanism 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 and the second assembly configuration is when the starter mechanism is spaced from the adapter to operate independently of the motor-generator.