Happy, I'm running a 2.2hp Mitsu with power pipe. It's a good engine for my needs; when I find optimum gearing, it'll climb my hills easier, run the flats and have enough low end for pedal starts. It's a versatile powerplant. If you want to gear it low for torque,it's good to go. If you want to gear it with 1.5" roller, it'll work with that. However, I have a hunch that this Mits engine could really use a transmission like the NuVinci hub.
I started with a mild-mannered 1.6hp Subaru engine, which I liked very much. It was a good match for my 20" Dahon. Top speed was 26mph with 1.125" and 190 lb. rider.
You have to remember that these are pedal-assist bikes. Unless you're running the NuVinci hub or super-low gears, everyone gets to pedal a small part of the ride.
That's the "exercise" part.