A replacement automobile engine is an engine or a major part of one that is sold individually without any other parts required to make a functional car (for example a drivetrain). These engines are produced either as aftermarket parts or as reproductions of an engine that has gone out of production.
Replacement engines are used to replace classic car engines that are in poor condition or broken, or to install a more powerful or more fuel efficient engine in a vehicle. Replacement engines are often used to make old cars more reliable for daily driving. Classic car hobbyists may also install reproductions of a rare powerplant in a classic car (this is most often seen in Mopar muscle cars that have the 426 Hemi installed into them).
Aftermarket engines are used in many forms of motorsport. Some late model racecar series use "crate engines" many of which are made by independent firms. This ensures that drivers all have similarly powered racecars. Legends and Allison Legacy Series cars also use sealed crate motors.
The four most common types of replacement engines are: 
New castings of some engines are sometimes produced by independent companies. These blocks commonly replace rare or popular designs for aftermarket rebuilding, especially when the original is no longer produced. They are sometimes available in aluminum instead of original iron, or in stronger alloys. Often they imitate the larger available displacements that were produced in small numbers or allow for displacements never available.
A crate engine is a new or remanufactured engine that is built with fresh components, such as bearings, rings, and lifters. You can purchase crate engines with or without cylinder heads. When an engine is mounted perpendicular to the ...