Is this safe? It's a cool idea and I'd appreciate even the slightest discussion of potential risks of the motor coming apart, or whatever else failure mode (I don't know)
The bigger issue is the terrible efficiency; alternators are about 50% efficient, whereas proper mechanical generators are 90%+.
The better solution would be to buy a "generator head" and connect it to the engine...but by the time you get done making a frame to hold the generator, engine, fuel tank, radiator, expansion tank, etc - as well as the necessary safety systems - you might as well just have bought an used generator.
The engine won't come apart in a dangerous way. It might overheat, which creates superheated water in the cooling system, which is designed to handle that. Although I sort of doubt it since they are basically using about 1/20th of its maximum power output.
The most dangerous thing about this is likely the standard risks of any generator, fuel and electricity.