Thanks to not so new concept conversion from petrol or diesel could be easier and cheaper then we tough ( in nearest future).
Instead of trying to fit electric motor and connect to existing power drive, remove all together and attach new wheels with inbuilt electric motor already.
Concept is so cleaver because also allow for broad competition between wheels manufacturer, regardless of any big car manufacturer willingness to capture or dominate the market.
You still may have your favourite brand car - Mercedes, Holden or Toyota but wheel with wide power range of inbuilt electric motor, you could choose like tyres today. Michelin looks promising and since 2004 when concept was presented the time is coming...
http://www.hybridcars.com/technology/protean-continues-campaign-wheel-electric-motors-30599.html