Those weight tables can be a bit misleading because of items not included. The installed weight of a Rotax 447 is 82 lbs with B-reduction drive, recoil starter and exhaust. The 618 will weigh 126 lbs with electric start (needed), and a heavier reduction drive (also needed). That total does not include the 25 lb battery to crank it, and the coolant! The difference in installed weight is close to 60 lbs. The 30 HP electric motor on Mark Beirle's Thundergull weighs 16 lbs. Most planes can cruise on half the engine needed to takeoff and climb. For the 618 you are carrying 60 lbs that you don't need for most of the flight. Jeron plans to run the same prop with both engines. You need a reduction drive anyway but will have to add a sprague clutch. Plenty of room in the cowling for the electric motor the new ones are tiny. Look at the Thundergull electric motor in the photo album for August 2011 Light Plane World. Hybrid designs may not replace ordinary aircraft real soon but you are going to see them flying pretty soon at airshows

http://www.eaa.org/lightplaneworld/issues/1108.html