DC motor power generation
They ought to be similar but depend on construction differences preferred for generators and those for motors.

It may depend on series or shunt, and method commutation, but a generator ought to be as efficient as a motor if the load is linear. I am assuming this is a brush-commutated motor.

If we say the motor is 90% efficient and dissipates 180W in heat at rated input then it can generate 1620 W of equivalent mechanical power output.

A simple analysis

Thus if you input 1620W of mechanical power input with the same torque and RPM, and losses are 10% it can generate 1458 Watts of electrical power output dissipating only 146 Watts of heat.  With 10% more torque assuming the bearings are still ok, then it can generate 1620W of electrical. 

Commutation losses depend on a resistive load and if not such as high capacitive battery load, then the conduction and commutation losses may reduce efficiency more perhaps required derating.

If the load is pulsed creating a back EMF on the commutation, then further derating and arc-related discharge ageing on brushes.

The major losses are core loss, copper conduction loss, eddy current loss, commutation losses.

Due to armature reaction, in motors armature and rotor flux cancels in the back iron so generators can have thicker back iron and thus greater hysteresis core losses.