Rebrickable link:
https://rebrickable.com/mocs/MOC-33398/AKM-Sky/piston-engine-flat-four/#details

Piston Engine Flat Four

This is not a "boxer" engine, explanation can be found here:
https://jalopnik.com/its-time-you-knew-the-difference-between-boxer-engines-1825246413
The model is very simple, can be done with your spare parts and used as MOC engine if you want something unusual.