My wording is a bit off, it should have said "flywheel " iso counterweight, an engine needs this too "smoothen the 4-strokes, when converting a car engine the mass of the standard fitted flywheel at the end of the crankshaft is often removed to save on weight because it is replaced by the weight of the prop, so if you want to run the engine without prop maybe ask Valley if that's ok.
Greetings