rabbid79 is correct that 146 ft-lbs of torque is required for 50 hp at 1800 rpms. Of course, if there is gear reduction between the engine and generator, you could run considerably higher rpms and produce that 50 hp with lower torque. However, that 50 hp is basically the MAX hp a non-turbo 1.6 is rated at and that is at 4800 rpms! I would not want to run a 1.6 for extended periods of time at max hp at 4800 rpms!
A 1.9TDI would be a better choice although would still likely do better with some gearing so it could get higher in its rpms. At 3600 rpms, that would be half the torque, so it would be having a much easier time.