Oooh, juicy argument
I'll just shove an oar in
When I started riding, other than going through the momentary madness of liking choppers for a month or two, the first style of bike that appealed to me was the streetfighter.
That was in 2002 and I think back then the idea was defined by streetfighters magazine as being a high performance motorcycle stripped of anything that was not absolutely necessary for operation, and then tuned up to make it even more lairy. This is almost exactly how I view cafe racers as well, but I think that a bike must have that nod to history, and a dead flat line from tank to tail, to be cafe racers.
Over the years even streetfighters magazine (and euro publications like bombers) have broadened their views on streefighters, often to accommodate styles from other countries and its these differences that have started to really define what can be called a fighter.
To start with I think the crashed and rebuilt bike is no longer a fighter as it is not built with the intent to make is something special: only the intent to get it back on the road, which is laudable and can produce some amazing machines (check out ratbike.org and search for survivals to see where that philosophy ends up). Repairing crashed bikes maybe where the fighter started, but it has evolved and developed a specific requirement.
In the UK most fighters were originally either sports machines or muscle bikes. Muscle bikes will have engine tuning but that jump to fighter-dom is the changes in chassis and suspension to make those big ladies dance. Traditionally sports based fighters were the direct descendants of cafe racers. focussing on stripping away the lard and also on allowing the tasty performance bits to show, which is why the fairing comes off.
German fighters nearly always had extreme bodywork but under the fancy engineering many of them remain only subtly tuned because of the TUV restrictions. The French used to be the kings of outlandish bodywork and chassis mods but again, often the engines are fairly stock underneath. America, well.... Their history is in drag racing and that shows in their performance customs, as does their love of bling. One thing all of these machines had in common was the aggressive design, something that was earliest and most apparent in the bikes coming out of the UK and Germany.
All of this influence has (with a lot of leeway) given us a profile of a modern fighter as a tuned motorcycle, sometimes modified to improve performance in engine power and handling but always having a modified appearance to make the vehicle look aggressive and unique. Aggression is the key, which really should come as no surprise when they are called street
fighters!
BTW, love the K100
I'm building a custom based on an '86 Kawasaki GTR1000, slow going as its my daily ride but its good to see someone else making something angry looking out of a tourer.