Can anyone put us straight please
The design criteria for DC motors and generators are completely different. For a motor, start under load characteristics might be more important, and, of course, it will be designed to operate at some design nominal speed. A motor used as a generator will
seldom produce the output its name plate data suggests. If start under load is what it was designed for, then it's probably a series connected stator. That makes for a generator with
horrible voltage regulation. Parallel wound stators, or permanent magnet stators, stand a better chance of working at least a bit right.
Most generators have stator coils optimized for producing strong magnetic fields. When used as motors, they typically don't rotate at over a few hundred RPMs -- if that's what you want, OK. If not, the results will disappoint (and probably require a stator re-wind).
Then there is the possibility of a mechanical failure. A DC motor will have to run at above its design speed if it is to produce the same output voltage (since its stator magnetic field will usually be weaker). Cutting down the magnetic field strength to increase speed of a generator might push it above its design nominal speed, if you need more RPMs. You could be looking at ruining the rotor by slinging off the rotor windings and/or commutator bars.
About the only exception is the starter motor/generator that jet and turbojet engines use. These are independent field devices whose characteristics can be tailored for either engine starting or power generation.