WOW! Unitrol 1020 on a cat C27, I never saw that AVR used on a generator below 1MW.
There are certainly many ways to achieve speed and voltage biasing on a C27. Newer model caterpillar, with EMCP4.3 and 4.4, speed/ volt biasing can be done with digital or analog( 0-10V) input via the EMCP 4.3...