Bro there is local bike market in Karachi at Akber Road near Saddar.... Largest bike market in the country (my opinion). Some times heavy bikes can be seen there.....
but there is no proper market for imported heavy bikes.... but i have seen two showrooms of heavy bikes in Karachi, one is named as "Malikoon' at shahra-e-Qaideen near Mazar-e-Qaid and 2nd is at Zamzama. As far as i know malikoon also known for repairing and customization and also sole supplier of heavy bikes to govt of Pakistan. "malikoon" is owned by a guy named "Pappu".
Thats all which i can contribute.....Let the other Karachities come and share their part of knowledge..