Manta,
I have not seen this done but have you considered a drive shaft? There are some shaft drive bicycles out there that do not use a chain from the pedals to the rear wheel but rely on a shaft with a ring and pinion gear in a housing at the rear hub. You'd have to figure out a way to mount a centrifugal clutch between the engine and the drive shaft but it sounds as if you're intent on this project and you asked for ideas. There's one. Just be sure to consider your rotation.
Tom

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