Onboard charging system for hybrid electric tractor
If you want to charge a 24V battery from a 12V alternator, you will need a huge boost converter, which will be spectacularly expensive.  It would be better to just buy an alternator designed for 24V systems, e.g. from a truck.

Yes, a battery can act as a short-term power source, which means that you can get a bit more power in the short term than the alternator can supply.  But you still need to pay attention to the thermal limits on the motor: 100A continuous or 300A for 30s (and probably off or very low load for a few minutes after a high-power burst, so that it can cool down again).

For example (totally making up numbers here) if you calculate that your average load will be 60A, you could use an 80A alternator.  It would supply the base load, and then when you want a 300A burst, you can pull that from the battery.  While the electric motor is cooling after its burst (e.g. drawing only 20A), the alternator will charge the battery.

With a motor of this design, voltage = speed and current = torque; normally you use a PWM speed controller which acts in conjunction with the motor's internal inductance to behave like a buck converter.  If you run it from 24V not 48V then you can put in only 24*100 = 2400W continuous (electrical) power; it will run half as fast on 24V (about 2400RPM) as it does on 48V (about 4800RPM) while providing the same torque (a function of the 100A limit).  If you want to get the full 4.8kW (electrical) / 4hp (mechanical) power, you need to run it at 48V and 4800RPM.

Note that the motor is rated as 4.8kW electrical and 4hp (3kW) mechanical.  That means its efficiency under full load is about 63%, which is terrible.  If you want 3kW of mechanical power, you need to supply 4.8kW of electrical power.

Because your choice of supply voltage affects motor speed, it will affect your choice of gearbox ratios.