You essentially want to match or exceed the performance of the top production e-motos pushing a heavier load than they do. I hope you're talking about flat terrain and relatively short range, because otherwise how can you expect to out engineer the top people in the industry with your first build?
Since you're looking at the weight of a gasser moto, start there with the power requirements for the performance you want. Power = torque X rpm, so an electric motor rated for the power you want is all you need, leaving only the correct gearing ratio to get you where you want.
More important than the motor type is what battery pack do you plan to use, because if you seek truly useful range, then the battery will be the heart of the system and is the biggest, heaviest, and most costly component in your system. Plus the frame needs to be built specifically for the battery pack.