The first thing you need to do is figure out what will fit onto the bolt spacing on your engine's exhaust port. Most headers on the market are designed to fit either Zenoah or Homelite. Not much available for the others. I used to run 21cc Echos, and the Zenoah (34mm on centers) spacing was a little too wide but could be made to fit with some light Dremel work. I think the Echo was 32mm. The Homelite spacing was wider still. Measure the bolt spacing and look for a pipe to fit that or one that can be opened up slightly to fit. One of the dealers like Marine Specialties may be able to recommend a header to fit. I don't recommend any system that uses a silicone coupler for a gas engine because the coupler won't hold up. Exception: If you run water in the pipe, the silicone coupler is fine. Wet pipes are quieter and run cool inside a deep-vee hull but aren't as fast.
Marine Specialties - http://www.marinespecialties-rc.com/ - carries several gas pipes for Zenoah and Homelite engines. Quickdraw pipes will crack over time, especially if the mounting is too rigid. They are also a fixed length. In adjustable length pipes the Hanson style pipes with the water cooled coupler are a good choice. Midwest Engines - http://www.midwestengines.20megsfree.com/ - carries a variety of pipes in this type. All of these pipes are designed for smaller, faster engines and will be a compromise for you.
I guess I got bored over at the gas message board. :-*
Back to the pipe question - I would guess that his 40cc Echo has relatively low flow volume and can do ok with the pipes designed for the hotter 23-25cc gassers. My biggest concern when running converted and freebie engines was not to break the bank on a pipe. But then money on a good pipe is money well spent. When I ran Echos, they liked the pipe to be on the long side (don't cut that header quite yet!).