Lightweight - go for it! This thing is really quite a lot of fun. In terms of see-through engines, the one that Matra directed you to is very good. I've also used Lexan to create "windows" into the crank case of engines that I've built for display only. I just cut out a rectangle in the engine block, and insert the clear piece. It allows me to see the crank shaft and pistons in action.
Henk - the guys at The Quail were not from a company. They were members of the San Francisco Scale Engine club. These guys build from scratch. I have a name and contact number at home if you'd like to get in touch with them. They really do wonderful work and are a great group of folks.