Sounds like the resistor serves as a make shift, field load. The field is electromagnet, with significant inductance. The resistor will not serve as an acceptable replacement, in a manner to eliminate CEL. It will just consume power, and get hot. I can comment more... If the field controller is FUBAR, then adding the resistor is senseless.
It should not be hard to follow field control circuit, via wires and identify the power transistor (IGBT) that is defective and replace it. It will likey be in a TO-220, DPak, or D2Pak. If the alternator was over charging, the transistor is likely shorted. That can be easily identified with multi-meter. Shorts also can lead to burned traces (open), so other repair may be necessary.
Yes external regulator works if properly wired.
