A Google Image search shows independent semi-trailing arms in the Metro, so—major surgery.
But I saw the link MetroMPG posted in
Permalink, about the VW Twin Drive. It has an ICE into an electric motor into the differential. The engine (no flywheel) runs at a single speed and the electric motor is used <30mph.
I can see that packaged in the form factor of a VW or Porsche 901 trans-axle.