This was a century ago, when aircraft engines were radial ones with propellers, and weren't all that different or complex than truck or tank engines (outside of layout and materials for weight optimisation).
Modern jet engines are technological marvels of engineering with extreme complexity. And there are massive advancements like turbojets, turbofans, high bypass turbofans, geared turbofans, open fan every few decades.
Any engine manufacturer from the 1930s could and did manufacturer plane engines. Nowadays it's so complex and capital intensive there are 3.5-4 companies remaining with modern efficient designs.
Airline became United, and engine manufacturer - Pratt & Whitney!