When I built here, I made my "genset" - a CHP - based on a 4-cylinder automotive engine (a ford laser 1.8 litre, 16 valve, overhead cam).
Removed all the fuel injection crap and the engine control electronics.
Added a performance mixer before the inlet manifold and modified it for operation off propane just like a commercial car conversion.
Also added a lambda controller in a closed feedback, monitors O2 in the exhaust and controls the fuel (propane feed) to ensure optimum AFR regardless of speed and load.
Built a micro controller to do auto-start, auto-stop, sequencing, cranking, monitor temps/oil/etc, run times and service intervals.
Added an electronic governor with a pickup on the 112-tooth ring gear that operates a linear actuator that drives the butterfly valve (throttle).
Stripped off all the extra junk - water pump, alternator etc.
I got a surplus/scrap tube-in-shell type industrial heat exchanger, milled the end caps out to make plenums either end, and adapted the exhaust to run through it.
My "cooling" water now has a 60-litre-per-minute grunfos circulating pump that pushes water through the exhaust heat exchanger, then through the engine block, then out to my large heat storage tanks (heat exchangers in the bottom), and back to the circ pump.
I have a vortex flowmeter in the water line so I can see how much water is flowing, and temperature measurement of "cold" return water and "hot" exit water from the thermostat body. This lets me fairly easily calculate how much heat I'm getting from the engine.
If the conversion of fuel to electricity is 20% end-to-end, for 8kW electrical output, I should be consuming 40kW of gas, which leaves about 32kW as heat. That much heat will raise the cooling water by just over 15 deg C at 30 litres/minute flow, and that's about what I see in real life.
If anyone wants pics, holler and I'll post them.