For those who have come to this thread only of late, we now have a post in the "Classic" section that addresses expansion chambers as an overview.

I've been searching to see if anyone still produces them as they once did when 2cycle bikes were common. Leo Vince hand crafts them but they are "pricey". If a good working pipe for a 50cc were cast to make stamping dies
then sheet metal could be stamped to create the two halves of the truncated cones section and tubing could be bent and all of it robot welded together in a jig in production to get the cost of these down considerably.

Yahoo! Shopping Search Results for Expansion Chamber

Expansion chambers are probably the simplest & least expensive way to add a boost in power that you can feel. A higher volume air filter works well when using a chamber too.

On a 50 to 100 cc motorized bike installation I'd think that routing the chamber upward on a diagonal slope to the rear axle (or further thru a fiberglass packed bolt on silencer) would have it up out of the way of the pedals and be comfortable for the rider. It would look like the old TT pipes of earlier times motorcycles too.