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.

It’s also less complex, with 230 components, fewer than that of the V8. “The Playz V8 Model Engine is a great kit for learning about engines,” said one shopper. “It's easy to build ...

Many kits of varying complexity are available ... You may need a small battery for it to run (although not all mini combustion engines require one), and you will also need lubricants, just ...