Production machines are mass produced, and whilst compromising various parameters. it is surprising how much of an improvement can be made by just setting up the engine to specification.
(For many shops, "near enough" is often "good enough"; and some can't even manage that, making quite incorrect settings. You wonder if those folk could diagnose a flat tyre!)
An easy "improvement" is to match the ports, manifolds, and joints so that there is little or no step between them. Be careful in changing the shape of the ports, sometimes an apparent restriction actually improves the flow and Turbulence into the cylinder.
Sometimes, a small change to the shape of the combustion chamber can increase power.
It used to said that a prewar tuner of Rileys, Freddy Dixon, could add 5 mph to the top speed of a car just by stripping and rebuilding the rear axle. he was, effectively, "Blueprinting" the components. He was also an exponent of inlet and exhaust tuning
On naturally aspirated engines, improvements to the exhaust manifolding, will allow the use of a "tuned"system, with bigger advantage, if the pipework is low back pressure (ie Straight Through silencer rather than baffled. This will remove the high frequencies, leaving the pleasant low notes).
But be warned, "Tha gets owt for nowt", tuning the exhaust or inlet system to get ram at one speed, may well damage performance at lower, often half of that, speed.
The same can be said of changing valve events by altering the camshaft. There may be more power at higher revs, but a lot less torque at low speeds. If the power band is very narrow, you will need a lot of gear ratios to keep it on songl
And some engines need some backpressure, especially two strokes. Otherwise you can have worse performance and higher fuel consumption! In an extreme case the fuel carried into the exhaust could eventually explode, and blow apart the exhaust system
Also, tuning the exhaust system may well need revised ignition timing to maximise performance.
Avoid over advancing, as this will raise rates of pressure rise, and probably peak cylinder pressures. Peak maybe just a short "spike" which gives little extra power but can play havoc with gaskets, pistons, and bearings, and in the worst case the crankshaft or crankcase!
Enzo Ferrari once said that the ideally designed race car would fall apart as it crossed the finishing line; but I don't think that we are interested in going that close to the limits.
Also, remember that higher power has to be transmitted to the road, so the gearbox and final drive need to be considered. And having got the machine upto a higher speed, it has to be able to stop, handle and to take corners, so don't concentrate on the engine in total isolation; you are super tuning the complete vehicle.
In every case, you are balancing output against reliability and durability. The original VW Beetle engine was very reliable, (it was virtually impossible to over drive it), but as the capacity and power was increased reliability decreased. Some folk advised that to prevent valve failures, and extensive engine damage, the 1303 should have new exhaust valves fitted every 30, 000 miles. Apparently, the quickest way to break a crank was to increase the maximum engine speed. In my opinion, that was NOT reliable
Upspeeding any engine will decrease the reliability and durability of components, possibly just by extending the running range into the region where components go into resonance, and overstress themselves.
Howard