Thanks everyone, it is beginning to look much more original than my 'wide' scoop.
That wide scoop had started as a nice, narrow opening to take an original style scoop about 40mm high, just to clear the rocker cover. It then needed widening on one side to clear the dynamo and engine top hose, and on the other side to both centre the scoop on the car and clear the front carburettor.
Then after many cups of tea and much staring from all angles and beard scratching I lowered the bonnet at the front to get the wheels looking 'right' in the arches. That meant that the sides of the scoop had to get a fair bit taller too.
The side bulge was added to clear the front corner of my alloy air box that was part of the methanol injection system, while the secondary scoop was added purely for decorative effect, as were the aluminium louvres towards the back of the bonnet.
What I ended up with looked okay, but it wasn't how I had envisaged it so it's mildly bugged me ever since.
Over the course of the last four years I've made a few changes under the bonnet which have now allowed a slimmer, more original looking scoop to be created:
The dynamo was 'rolled' down and a new bracket fabricated so it's out of the way completely;
The methanol injection system was removed - even though it really worked when it worked, as the engine isn't mega powerful the small quantity of methanol/water mist required meant that the nozzle was so fine it kept blocking despite the filters in the plumbing;
I've raised the rear gearbox mount by 20mm, which dipped the front of the engine down by 10mm (that mod copyrighted by Phil J);
I cut the elbow off the thermostat housing hose connector which frees up a surprising amount of space to the side of the rocker cover.
Taken together, those changes have removed a lot of the obstacles under the bonnet and mean that only the rocker cover and the very edge of the top hose need clearance now, and the new scoop is taking shape nicely.
As for the wing scoops on the original D24's, the answer may well be lost in the mists of time. Only two of the six built survive and no two look the same in period pictures.
Lancia clearly believed in developing their cars on the track - including hacksawing the roofs of the D20 coupes when the drivers complained that the cabin was too hot - and the rows of solid rivets holding the wing scoops on suggest that they were added after the bodies were finished by the coachbuilder.
Additionally, there are a number of 'continuation' models out there that were built in the 80's or 90's (using original engines and factory blueprints), which have wing scoops but may or may not have the additional widgets beneath that they serviced.
I guess a visit to the Lancia museum (if I can work out where it is) the next time I'm in Italy is on the cards to try to find out more.
Oh, and while I was looking for details of the continuation cars I came across this article about recreating a Lancia D50 -
How the other half live...