"According to our designer You can use only our exhaust system on Your Sportstar SN2004 ****. We can You offer all new version E6 18-15 01 set of exhaust system or new exhaust pipe Front LH."
It was worth giving it a go. Its complet BS of course, with the sole intention of keeping your dollars in their hands. There can be no practical justification (other than greed) for not using say a Rotax OM system, which the installer can custom fit to the space/location available.
A word of caution on just replacing an exhaust bit here or there - a failed exhaust pipe can be a very dangerous situation in the air. Super hot exhaust gas potentially melting fuel /oil hose/wires = fire. Be sure to assess the whole systems condition.
A sort of posative; you now know that Evektor culture - they will attempt to skin you alive for every OM part they supply. 😈