Control of the field is how it's normally done. First take a look at
Electronic Components Distributor | DigiKey Corp. | US Home Page and search for ACS750 and LM395T, because these would be the componets I'd start with. The sensor will provide isolation and filtering for 75A so that's done. The LM395T is a Ultr Reliable Power transistor. It has a VCE(Sat) of 2V, so you would need an 18V (2V above 16)and a 5V supply at aminimum. I used a 75A transducer so your not at the limit of a 50A transducer.
An Op amp to scale the current (Scale), a voltage reference IC (Ref) and another OP amp (control). That transistor needs a very minisule amount of current on the base and it's protected all around thermally and short circuit. The control OP amp will have the reference and the setpoint attached to its inputs arranged such that the transistor is modulated until the setpoints agree.
This is just a verbal description and not a design at this point. Take a look at the parts.Find an available one andlook at the datsheets.
I do think that one thing is missing from your conception though. You have to guarantee the field is OFF while the engine is cranking which means sometime after you have oil pressure.
I'm not sure how the RPM of the generator is controlled either.
Usually, there is another winding on the genertor that generates some control voltage from the residual magnetism and this winding creates the voltage for the field. Once the system is producing, those voltages and the availabe currents increase. This also might complicate the control.
If there is a battery, which there probably is, the extra winding may not be used, but something is controlling the frequency. DC to DC converters could be used to generate the power for the circuits to operate.
Remember that there are overvoltage and undervoltage, frequency, water temp,oil pressure that are all part of the system.