A motorcycle engine sold as a complete functioning unit is referred to as a crate engine. A crate engine is often used as a replacement when the original motor is in poor condition or to install a more powerful engine in the motorcycle.
The use of crate engines is popular with classic bike enthusiasts and racers as an alternative to time-consuming rebuilds or expensive modifications of existing engines. The name comes from the fact that a replacement engine is delivered in a crate with nothing else required for operation.
Established engine models with long production runs in the same configuration are the most likely candidates for production as replacements. Harley Davidson sells new crate engines for many of their popular models, and S&S Cycle makes replacement motors for Harley Davidson frames as well as a range of performance crate engines for custom applications.
Not all bike motors are available as crate engines. Motorcycle manufacturers such as Honda, Kawasaki and Ducati change engine configurations and capacities so regularly that no single engine block has gained enough of a market following to warrant production as a replacement. In this case, an individual engine must be rebuilt rather than replaced with a crate engine.