Model originally uploaded to Thingiverse at https://www.thingiverse.com/thing:4544716.

Dummy_9_Cylinder_Radial_Aero_Engine

This design is not based on any particular real world engine but to look something like that is easy to print as it doesn't require support. 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.