DC generators began as brushed commutated devices. They had a one or more stator windings and an armature winding. Field wound DC generators as well as motors were commonly connected in one of three methods: Series, Shunt and Compound. Without getting into details, each had its own set of strengths and weaknesses. But you only have to remember these two things: the voltage of a DC motor is dependent on its input shaft speed. Current is a function of torque. More voltage means more RPM's and more amps means more newton-meters....
So with all that, I need a constant speed source to get a constant voltage. And I need to ensure I have enough torque to satisfy the current demand of the load otherwise voltage drops off. Old vehicle had commutated generators. They couldn't regulate the voltage so they used a range of around 10-14 volts and used a relay that simply closed when the engines speed was within the voltage range. If the voltage went too low or too high, the relay opened. Primitive by today's standards. The Alternator in today's vehicles uses a voltage regulation circuit that varies the armature current which changes the field strength based on the stators output voltage. Lower speed means more current to the armature and less current at higher speeds.
So how different were DC generators from motors? Not very different at all. If anything they mostly differed in mechanical design as they were to be coupled to a prime mover (steam, ICE, electric etc.). Well, firstly the dynamos do need either some kick-start power or permanent magnets, as they can't depend on electromagnets in the stator only; I know that I won't generate any electricity by moving a wire without any current next to another wire without any current. A permanent magnet DC motor will act as a dynamo if I provide rotation power to its axis, so there's not much difference here. The commutator may be aligned a little bit differently, ignoring the need to cut off early, when the magnetic field would brake the motor; there would be no concern about not creating a 'dead zone' where the motor wouldn't start, not pulled by a neighbour magnet. If anything, the device would be considerably simpler. Now if I want to engage electromagnets for the stator, there will be some differences... usually they would be powered from an external source, which could be then charged from the dynamo.
Though, in much larger dynamos they had adjustable commutator brushes to compensate for the shift in the commutation plane as a result of heavy load characteristics. A hand wheel would turn a worm gear which would advance or retard the commutation plane to bring the generator back into its normal operating parameters. I'm guessing that it's the nameplate RPM which I need to spin the motor at to get the nameplate voltage. This means if I have a 12V motor that spins at 6000 RPM, I'll need 6000 RPM to get 12V. If I don't have a constant speed source then I have no way to regulate the voltage. I would then need a buck-boost switching regulator to get a constant voltage from the motor.
DC generators, or dynamos of any significant size are rather rare these days. It's much more common to use an AC generator (alternator) with an external rectifier. And just for reference, an AC motor can also generate power if you spin it faster than its nameplate RPM, usually at synchronous speed. But again, no voltage regulation and a constant speed is needed. More trouble than its worth. Also of note: jet planes use a very elaborate mechanical speed regulator to produce constant shaft speeds which ensures a constant 60 or 400Hz AC frequency as the throttle is varied.