It would be very interesting to know just where the weight is in the car, It can't be the engine, the body is ali, the wheels shouldn't weigh too much, the gearbox is a bit of a lump, but that on its' own shouldn't drastically effect the overall weight. Is the chassis made of solid iron?
I would be curious to know how much the total engine/drive train package adds to the 3 wheeler.
American motorcycle engines are not known for their light weight and the big capacity V twin produces a lot of torque that probably has required heavier parts to be used in the construction. Incredibly Morgan have chosen an engine that has a similar capacity and power output to a 4/4. The gearbox comes from a Mazda car but has a 90 degree drive fitted to the back of it to use a final belt drive. It would not surprise me if Quaife, for example, could not have produced a nice lightweight sequential gearbox and final drive that would have weighed less. Obviously a motorcycle gearbox is out of the question because it lacks reverse.
It seems to me that for 1000 units a year it would not have been impossible to find a European manufacturer. Forget my Moto Guzzi bias - there is also Rotax who currently produce a modern 1000 cc engine that produces 106 hp, has a 5 speed box with reverse, and is produced for the Can-Am Roadster (trike). Ok the Can-Am is more bike than car and only weighs in at 317 kg but that still gives more than 130 kg to play around with, to still come in under 450 kg, for the larger Morgan frame and bodywork. Probably that particular motor and drive would not be suitable for the heavier Morgan but I am sure something could be organised.
I suspect that the choice of mechanicals was dictated by the origins of the design and the desire of Morgan to crack into the huge American market. In that market the American style engine is no doubt more acceptable than a European engine.
I find the design fascinating and think Morgan have a huge success with the 3 wheeler (I would love to own one) but I can't help wondering if they may not have been better off producing something a little less potent with resulting lighter components.