Reddit reviews Making Simple Model Steam Engines

Not so ...

Engines have been around a LOT longer than the internet. Before you could google anything, you could find books on the subject. There are lots and lots of books on model engineering and building small engines of various sorts. I have some books on building miniature steam engines that are over 100 years old.

This one isn't terribly old:
https://www.amazon.com/Miniature-Internal-Combustion-Engines-Malcolm/dp/1861269218/ref=sr_1_1?ie=UTF8&qid=1539635163&sr=8-1&keywords=building+internal+combustion+engines

And it's a pretty good book - not really a beginners book or detailed how to get started book, but good nonetheless. As others have mentioned, I'd recommend building a simple steam engine first.

Here's a good book for doing that:
https://www.amazon.com/Making-Simple-Model-Steam-Engines/dp/1861267738/ref=sr_1_1?s=books&ie=UTF8&qid=1539635352&sr=1-1&keywords=model+steam+engines

It is more of a beginner's book with complete plans for several simple engines.

Good luck!