A small generator could work, but their power output often isn't very clean. You'd need one with a constant load capacity within the limits of the i3's ability to restrict power using the low-power menu. The supplied (in the USA) EVSE is a maximum of 12A (12*120vac=1440W continuous), but you can tell the car to restrict what it uses - I don't remember the steps. As long as the generator could provide at least the minimum step, and the power is not too noisy (the EVSE may not like it and issue a power fault), you'd have a chance. Given the community average power usage is around 4m/Kw, you'd get about 5-6 miles/hour of charge at 12A/120vac. Throw in the range loss of carrying the thing around (similar to the REx), and that should tell you what to expect.