With a single cylinder 4T engine the right diameter and length of the pipe from the head to the muffler is quite important... (but after this piece of pipe you can probably do pretty much whatever you want, mufflers seem optional from a power point of view....)
The length of this part determines at what rpm the low pressure "echo" of the high pressure pulse from the opening of the exhaust valve helps emptying the cylinder during the valve overlap part of the cycle.. And you want this to correspond with your camshaft timing if you want to have maximum power....
The diameter determines the speed of the exhaust gasses and therefore the amount that the inertia of these gasses helps emptying the cylinder. Here, the trade off is between the benefits of exhaust gas speed and the losses from flow resistance.. The optimum is dependent on displacement and, again, camshaft timing...
A larger diameter or shorter pipe will move the peak in the torque/rpm graph to a higher rpm, but when the camshaft timing isn't altered, it might as well lower this peak in strength, probably also lowering your peak power even while it's at a higher rpm...
So : unless you have a non-stock camshaft or a BBK you should keep the length and diameter of the pipe between the head and the muffler about the same as stock, assuming Kymco has correctly tuned these parts to work together, which you will only find out by trying something else of course...
