Sorry for the delay, yesterday my account was hacked and it was impossible for me to answer your questions.
I would like to know the power involved in this project, KW, BTU/ hr. and also, how do you think to capture the engine's heat.
About the generator heads: You can obtain a linear response with respect to the RPM's and the output voltage and frequency (Hz), using an external, fixed but adjustable, DC supply. In this way, if the heat demand increases, the RPM will increase too, doing the same the output voltage. This applies to a brushed or to a brushless generator. For a permanent magnet generator, this is automatic, more RPM's, more output voltage.
If you mention "Generator head", it means that an engine will be coupled to it, directly (same RPM) or using belts and pulleys. The first option fixes the generator speed range to the engine's speed. The output voltage will vary the same. With a high excitation level useful electric power can be obtained at low RPM's. Increasing RPM's, a high voltage, higher than the max solar regulator can withstand, will be produced, damaging the controller. This implies the use of a limit speed governor, set to obtain at the max engine RPM's, the max voltage allowed by the solar regulator-inverter, adjusting it with the external DC supply.
Just to mention, if you use a diesel engine, (for prolonged periods) you may consider for your calculations that, approximately, one third of the power obtained is converted in mechanical power in the output shaft, one third in hot exhaust gasses (400-500 °C) and one third in cooling water and engine irradiation.
Well, I hope this help something to your project.