An AC induction motor exhibits a fairly flat torque curve from something like 10% of design RPMs at 50/60 Hz, and then is usually shown as decreasing, because, although motors can be driven by PWM to several times their rated speed, it is not usually recommended (or feasible) to increase the voltage accordingly (as a VF drive does). But there is nothing "magic" about 60 Hz as a limit for the magnetics, and it is possible to design motors that run up to at least 400 Hz. They are typically very high RPM, but with enough poles, it is possible to boost the HP of a motor by several times, using lower voltage windings and running at least up to 150 Hz. You can get 2 or three times the HP from the same size motor. This is very important for highway vehicles, where the weight and size of the motor contribute a lot to fuel economy and performance, but probably not as much for a tractor, where additional weight might be a good thing.
Since large induction motors are typically 92 to 95% efficient, a 75 kW 100 HP motor will produce something like 5000 watts of heat, which is removed by means of self-contained fans. A motor specially designed to be overdriven might be even more efficient, although there is a limit where magnetic losses take over. The good thing about electric motors is that they consume no power when they are idle, and their losses are at worst a percentage of the actual output power, and may even be less when lightly loaded. Losses are proportional to I^2, while torque is proportional to I. They can also be "pushed" to 2 or 3 times their nameplate ratings for short periods of time, so you can often get by with a smaller motor if your power needs are intermittent.
So the transmission requirements are mostly to provide the needed torque, and then the motor speed can be adjusted as needed. Large tractors probably have trannies with 10 or 15 speeds or more, while an electric motor might require only 3 or 4. This would be another saving. VF drives are so efficient and inexpensive now, that any other motor controller is just about unthinkable. And you can run a VF drive on 720 VDC directly, so it is ideally suited to a battery pack for use when transferring from one power source to another. This would require much less power than the tractor is actually rated for, so the battery pack could be quite small.
Paul