Ah that looks like really good fun, low power engines and cheap to build! I envy the joi de vivre in your countryside, the suburban nimbys have just about killed most powered countryside sports here.
My pedal car body was made of polyester/glass cloth over a polystyrene form covered with building mortar mould then painted and waxed. This was then broken up to release the shell. The frame is light steel tube and bicycle components. It was too heavy to be competitive, but it won a Concours prize in 2010 and also the prize for the oldest team. Pedalcar racing is a young person sport and very competitive, there is even an annual 24hr Le mans type event.
Trevor