Hi,
I think it was essentially "parts bins engineering" -- they already make a 1.4L 4-cylinder. They would have had to design the smaller engine from scratch.
Here's a purpose made "range extender" that is damn big -- 35kW driven by a 1.2L 3-cylinder:
Green Car Congress: Lotus to Introduce Range Extender Engine. That should be able to run two or three Volts?