I bought a 20” 250W front wheel to fit to my Kettwiesel and it goes uphill like the wind... from being a brisk climber as a trike it now goes like a rocket and I was very surprised to find myself claiming KoM on a nearby Strava segment. (I promptly deleted it as I didn’t feel that was fair competition.). As you’ve been musing, hills are a bit of a challenge on my Fuego too and I have thought of trying the wheel out on the bike just to see what it does. The only issue is that most of the time my unassisted Fuego cruises well over the 15.5 mph legal assist limit so I’d be carrying around the extra weight of motor, battery and electronics just for the relatively few troublesome hills; the motor is much more useful on the generally slower trike.
If you do think of fitting e-assist to the Fuego I reckon mounting the battery under the seat would have the most minimal effect on handling and stability; my 13 Ah battery is pretty heavy and mounting it outside the wheelbase or high up would compromise the Fuego's excellent dynamics.
