The BMW i3’s range extender is based on the BMW C650 Scooter engine. In the scooter version, the engine is rated at 65 hp, but it is de-rated to 35 hp for use in the i3. This could be partially because it will be running at lower RPM for maximum efficiency rather than looking for maximum power output. It could also be that the valve timing or some other part of the motor has physically been changed to increase fuel economy for use in the i3.
BMW likely went with the C 650 scooter motor because, since scooters are a step-through design (rather than step-over like a motorcycle), the motor has a much shorter height overall. The front cylinder is sloped at a 70 degree angle from vertical, making the engine very flat. This allows it to fit under the rear deck of the BMW i3 without protruding into the cargo area and diminishing cargo space at all.
The BMW i3 will have an electric range between 80 and 100 miles and is ultra light-weight due to its carbon composite design. 0-60mph will be around 7.2 seconds, making this little EV quite the blast to drive!