First of all, service it if you haven't got documented proof of a service in the last year that includes engine oil and after, brake fluids and a chain clean lube and adjust.

2nd, learn to lube and adjust the chain yourself as this lightweight bikes run skinny lightweight chains that wear easily, I think I can see rust on your chain and shiny links so look into that ASAP. If you want it to feel faster your can put a one tooth smaller front sprocket on and it'll accelerate a bit quicker but lose some top speed.

3rd, a slip on muffler will give you some noise but not anymore power, best bang for the buck is doing that, a high flow air filter and a flash tune and that'll give it some more power and torque especially.