A engine like a Ryobi 31 type is long and narrow. There are certain airplanes that would require this.

Other engines like the Mac require almost no cutting if you put C-H ignition on them.

Some engines like the Homelites and the Ryobis are well documented for conversion.

Some engines have a better power to weight ratio than others.

Some engines require very few changes, like using the original ignition and carb to make a strong running engine.

Enjoy,

Jim