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bigscoot,
To my mind, the easiest way to get 50cc from something like the popular Lifan 140cc engine is to delay the intake valve closure, which reduces the displacement. However, this also decreases the compression since compression only starts when the valve closes. This can be compensated for by shaving the head to get what would be a disastrous compression ratio if the intake valve closure were not delayed. (There is a limit here as the valve has to get out of the piston's way or the piston has to be designed to be the closure mechanism for the valve.) If you have good machine shop access, this might be a viable answer for the project.
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