I don't know who the source is, but I can catagorically state that it would be impossible to fit the AM V12 engine into the engine bay of the Cygnet without extensive modifications.
The V8, however, would fit, longitudinally, with a shortened torque tube and prop, ideally connected to the Graziano manual 6-speed box. In order to minimise modifications to the floorpan in the front footwells, you have to push the engine forward such that you would have no space for a radiator in the conventional place, in front of the accessory drive belts. The radiator could be placed above the engine, but it is a tricky ask - the most effective cooling is always direct air. Ducting is always a lossy exercise.
This means you could retain all the existing suspension hardware at the front. At the rear, you would require a customised version of hte AML rear subframe, narrowed to make the car look more or less stock, and whatever wishbone/hub assembly you'd like to run. Handling of such a car, with McPherson strut front on lightweight hubs - probably not good. Nor wise, from an engineering point of view. You could build and weld in double wishbone mounts and put AML front suspension on, but you'd have to cut into the inner wings quite a bit - there's a lot of fabrication there.
However, if you don't care about the floorpan and you cut it to bits, fitting the entire V8 drivetrain in a more conventional way (say, into a tube chassis) is relatively straightforward, and if you can't be bothered to narrow the rear, then you could make some massive rear arches and retain the Vantage track. This would be a quick and dirty way of making an AML powered Cygnet - although it would look like a right monster, it would handle like an extremely short wheelbase Vantage. Kinda twitchy.
Would anyone build this car? Oooh, I couldn't say.