I have no actual knowledge on this, but after my first thought of the generator looking a bit too "new", I started looking a bit closer and have decided that it does have the potential of being original.
My primary reason for thinking so is that there would be no other likely reason for the back side of the base to be large enough to accommodate the equipment that is now there, unless it was intended to provide space for something similar. Also, the size of the crankshaft drive pulley (which may or may not be original) is also appropriate for gearing up the needed amount to properly drive a generator.
In other words, why would they provide such a large blank area behind the engine and boiler, and such a large drive pulley, if something like what is seen here was not originally intended?!?!