I went down a rabbit hole lately and found this article on tiny diesel engines built throughout the past decades, most of them model engines: https://modelenginenews.org/techniques/minid.html

> I've seen collectors turn a greenish blue and stammer out crazy offers on sight of my 0.1cc Nano, pictured here. But as the size goes down, construction difficulty and criticality of "fits" goes up exponentially.

Some Model Engineering related resources:

1. Engineering drawings of small model engines used for model aircrafts. They are designed for manufacturing and working, not not like showcase item. We can actually build them.

https://outerzone.co.uk/plans.asp?cat=Engines&Xcardsperpage=...

2. https://modelenginenews.org/midge/index.html