On a motorcycle, the engine is everything. It's not like a car where the works are hidden behind a hood (or two); the physical attributes of the engine dictate frame geometry, cooling, fuel consumption, powerband, acoustics, and of course appearance. People can develop strong opinions.
|Photo from Cycle World|
|1930 Indian Scout. Photo from Silodrome.com|
|1953 Vincent engine in a 1973 Ducati frame: Vincati|
|Walt Siegl Harley-Davidson Shovelhead "Rivera"|