I would also suggest avoiding a motorcycle engine as the characteristics do not suit the car. I am no expert but there are many modern small engines out there that would fit the eco/light/modern brief.
Nawww...the original three wheelers used bike engines I think.... so how about a nice modern big twin like that on the brand spanking new Ducati 1198...
"THE NEW 1198 TESTASTRETTA EVOLUZIONE ENGINE
The Testastretta Evoluzione engine is the crowning glory of Ducati's development and perfection of the L-Twin engine. World Superbike dominance for almost two decades is the result of continual commitment to their twin-cylinder configuration. Ducati now share the reward for that commitment as World Championship-winning technologies flow from the factory race team into production engines.
The incredible 1198 and 1198 S are powered by a liquid cooled, L-Twin, Desmodromic engine that produces 170hp (125kW) @ 9,750rpm and a class-beating 97lb-ft (13.4kgm) of torque @ 8,000rpm."
