Great question — answering as the UphillTech team:
Generally speaking, no. A liquid-fuel engine wants to turn a lot faster than a cyclist would turn the cranks. That's why a retrofit engine often has a small drive cog and an enormous cog on the rear wheel, to decrease the revs and increase the effective torque. The optimal speed of a small capacity liquid engine is somewhere between 1000 and 2500 RPM. A cyclist would do 60-90 RPM on the crank, with 120 being possible-but-abnormally high. If you want a motor bike, its often cheaper to buy a 50cc motor bike and leave your normal bicycle unmolested for riding. Also, if you remove the ability to pedal, then your ride becomes a motorbike no matter what displacement limits you're under....
Full answer by Criggie: https://bicycles.stackexchange.com/a/76136 (CC BY-SA 4.0).
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