The $29.99 all-metal desktop Stirling engine kit is one of the simplest. It can be assembled in 15 minutes and is recommended for ages 14 and up. Also for $29.99 is the Exploring Kid Steam Engine Kit.

If you want big power but can't afford the performance upgrades today, you can start with a stock motor and upgrade later as finances allow. Almost all DIY kit car manufacturers will provide or sell ...