You could theoretically get rated amps out of the alt, in real life they only provide rated current for a few minutes before they heat up. micro hydro has been using delco 10si and 12si and large frame motorcraft alts for years in 12,24 and 48v for many years, They commonly use a rheostat to control the field. You could build a pwm field controller for any volts you want, but remember those alts are lucky to be 50% efficient and take lots of power to drive. 50a @ 50v is 2500w and would take 7+ Hp to drive. The large frame motorcraft alts in the 70-100a range for a 70s full size ford or Lincoln are good choices and still fairly cheap and available and externally regulated. The claw rotor can take more than 12v, but the pwm duty cycle could easily be limited to 25% duty and the rotor would "see" 12v, and probably be more field than you need.