Model originally uploaded to Thingiverse at https://www.thingiverse.com/thing:5368337.

7 Cyl Dummy Radial Engine

This design is not based on any particular real world engine but to look something like that is relatively easy to print. This is 138mm in diameter but can easily be scaled up or down in your slicer software. It's been successfully printed at 90mm and 190mm diameters.