If you're building your own light-sport aircraft, plan to spend one third to one half of your budget on the engine, sometimes called the power plant. The exact cost depends on how much of a mechanic and a machinist you are. You can buy ready-to-install power plants designed specifically for light-sport and ultralight aircraft or you can modify smaller automotive and snowmobile engines to power your plane.
Propellers are easier to choose because LSA regulations limit the type of props you can install on these aircraft. We'll take a look at props after a discussion of aircraft engines.
Engines are engines. However, there are some major differences between the engines that power cars and ones that power airplanes. The greatest difference is redundancy.
If an aircraft engine goes out at 3,000 ft AGL (above ground level), it's not as though you can pull over to a cloud and call AAA. You'll have to land somewhere—hopefully safely. That's why most aircraft use engines with some redundancy. For example, larger aircraft have dual-magneto systems with two magnetos, two wires, and two spark plugs for each cylinder. You actually fly using both systems, but if one goes out you can switch over to the other and continue flying, albeit at slightly lower efficiency. Many light-sport aircraft use dual-magneto systems, too, for the same safety reasons.
Two types of engines can be used: two-stroke and four-stroke. Smaller engines such as the ones on lawnmowers often are two-stroke engines, simpler in design, lighter, and cheaper to buy. Four-stroke engines are more complex (your car has a four-stroke engine) and more expensive, but develop more power. Many ultralight aircraft are powered by two-stroke engines. Sport planes can be powered by either type of engine. The decision typically is based on cost; two-stroke engines are simpler and less expensive.