You can see the Hatori headers and pipes/mufflers on the following site:
http://www.hatori-models.co.jp/htdocs/data_e/top.html
Click "Airplane" at the top and then click the "4 cycle 48-140" section.
I looks like they suggest the "Cooling Header" # 661 and the 660 muffler.
Sam
Oxai.us