This is a super cool little mini-engine model, kudos to the person who originally designed it, and the two people who added some key improvements to it! Please note I am contributing nothing here other than making it clear and easy for the next people stumbling on this to print a great solution, organized in one place.

I wasted an iteration and some time figuring out what to do and how to make this work, so I want to share this here. All the pieces are separated if you need to reprint something. It uses the larger optimized "Better fit piston head" and the "Crank and axis stop..." which add a hand-crank option, and the really important "axis stop" to hold everything in place without any glue.

I also provide them on one plate STL for easy scaling and printing, at 75% scale it works great and takes just 2 hours on my Prusa i3 MK3 at the default Slic3r settings for "0.2mm Quality MK3", changing only