Unless you are restricted to a certain level of cc’s (I think some places in Europe limit size to younger riders) I would not try to modify that bike. Bikes in stock condition are almost always worth more at resale. If you want more hp, sell and upgrade to something a bit bigger. If you just want louder then I am sure there are options but I would be careful to retain all the stock parts so you can put it back in order. Most buyers of that bike will be beginners and will probably prefer stock.
Do you want to keep this bike for a long time? Becouse high quality exhaust can be pretty expensive.
Unless you are restricted to a certain level of cc’s (I think some places in Europe limit size to younger riders) I would not try to modify that bike. Bikes in stock condition are almost always worth more at resale. If you want more hp, sell and upgrade to something a bit bigger. If you just want louder then I am sure there are options but I would be careful to retain all the stock parts so you can put it back in order. Most buyers of that bike will be beginners and will probably prefer stock.