I don't think hoar frost ever forms on wing surfaces in flight, its fine crystalline nature wouldn't be strong enough to form or remain there. Icing on leading edges and anything with/near a stagnation point is another matter but that is nothing like hoar frost.
Hoar frost on wings usually forms on the ground after a cold-soak but I doubt WWII bombers would have returned with much fuel left and in modern aircraft with integral tanks the amount of fuel present seems to affect the severity of frost accumulation (more fuel stays cold longer). This would surely be a much less likely event with bag-tanks as the cold fuel is not in contact with the wing skin.
Airframe icing melts faster at higher speeds, not lower ones (due to higher TAT) so WWII a/c being slower would be far more prone to airframe icing than modern ones. Even so, this isn't hoar frost.
Re battered cowlings - why not give it a shiny new one (or in primer) as though it had just been replaced due to damage?