I addition to the above your options would include torsionally soft or stiff. I haven't built a generator/engine machine so I can't tell you how they are normally done.
For a generator? No way. The engine needa direct control of speed. For low horsepower? This can't be expensive. The Dodge Paraflex couplings work well here. Falk Grid Couplings work well but are expensive and require a lot of maintenance.
Never coupled to internal combustion engines, but have experience with fluid couplings on conveyor drives. Fluid coupling mfrs indicate good results in coupling to IC engines.
