The magic 3.3 mm Cobalt drills arrived this morning so I could get on and fit the grilles.
I did manage to snap one of them despite extreme care - the inner face of the casting has a steep taper, so one flute of the drill breaks through before the other is ready to emerge and thus grabs and snaps, but six holes drilled and tapped at the expense of one lost drill isn't too bad considering how unbelievably hard this cast iron is.
Now the hunt is on for a suitable housing for the generator. A 15 foot container would be just about ideal, but I don't think they come that length - tens, twenties and forties are usual. I need an internal space of at least 13 foot x 6 foot but within reason more is better.