The pusher doesn't have to be able to do all the work, if it just does half, you'll double your range.
Find a wrecked scooter what has the engine is integral with the swing arm and you could make a small one pretty cheap and see what it will do. Something like this.
2007 Complete Yamaha Vino scooter 125 engine assembly only 647 miles
125 might be a little on the small size, 8 hp. Those things are made to run full throttle most of the time so I doubt it goes rich. 8 hp = 6 kw
I found it takes 9.2 kw to go 45 mph, 16 @ 55, and 22.7 to go 70 with a Leaf. So a 250cc might be better suited to double your range. Go real big and use it to charge the Leaf battery by riding the brakes then shut it off.