If you want more power then you need more fuel, more air to get into the engine and relatively simple improvements to the induction system coupled with an ECU remapping to take advantage can transform an engine. An example of what can be acheived is given below.

Steve's 390SE has had an enlarged throttle body, and a Jaguar air flow meter and fuel injectors fitted along with a Mark Adams chipped ECU. The result was quite spectacular with quite considerable increases in response and power. Steve's car was quite fun on the rolling road as it was wheel spinning despite the tethers and a burly mechanic in the boot. The resulting power curve is like a map of a mountain pass with all the hairpins, as the car broke traction as the power increased...

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The modified injection system provides about 245-250 bhp (estimated based on smoothing out the dips in the curves) which considering the engine has not been mechanically touched, apart from normal servicing, shows how much extra power can be obtained from a relatively small engine size.