The ST250 Flow Kit is an aftermarket upgrade that includes a larger diameter manifold as well as a straight through head pipe.
The Header Pipe will require a muffler, Most people buy the Cafe Racer Value Bundle from the braaap website which includes a muffler. The reverse cone muffler is also a good option.
The manifold is slightly larger diameter, depending on your carby choice you may need to drill out the fitment holes to 10mm so that it fits the Mikuni Style or Standard braaap carby.
They don't come drilled out unless you order and pay for fitment :)
For optimum performance use this manifold as a base for match porting the intake from the carby through to the head.