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The engineer in me can't let this one go by.

A gear system can only reduce (i.e. waste) power. What it can increase is torque or linear force at the expense of velocity (angular or linear). So a 50cc engine could move a large train, but very slowly.

If you want extra power out of the small engine, then you have to increase the fuel flow rate, and air flow rate to go with it. A forced induction system at 10 bar (ten times atmospheric pressure) makes a 50cc engine act like a 500cc engine. But then you also need to cool it like a 500cc engine, and it needs the rods, crankshaft and gearbox of the larger engine to handle the forces and power, so by the time you've finished, you still have a large engine, but with very small cylinders.