Does it not vary by state?
In the UK, you don't need indicators or mirrors. The only requirements that I can recall (provided that you only use it on the road during daylight hours) are a horn and a brake light. Let's call that 5 lbs including wiring and button.
The biggest thing for a production motorcycle in the US is the EPA laws, followed by the DOT. There are requirements for how long the engine has to meet emissions requirements, in terms of operating hours. I really like the idea of the 450 dirt bike based racers..... seems like that would be the least painful transition, as they are at least in general relatively high volume production.
But as far as I know they have not been certified to meet emissions over the long haul.
I know in my heart of hearts that a small bore, very high strung bike would be a hoot on the street. For the US the perception is that the cost of that bike would be so high as to make it a non starter.
If someone were to enter the US market with a sub 600cc bike that weighed less than 300 lbs, with the same acceleration and handling as a current gen 600..... it would still not sell well, particularly if the price was anywhere near what the current cost of a 600 is.
Due to the realities of the legal environment here, no mfg is going to advertise how well their bike accelerates, or corners..... stripped of any way to tell the public why this is the route to go to have a performance bike -- not only does it handle better, but it gets better MPG, and is easier on tires and brakes, not to mention the services on a single or twin are less than on a multi cylinder motor..... but it costs as much or more than the bikes it's replacing..... dead in the water.
Unless someone goes the pirate route, and the mfgs take notice.
"Basic stuff fellas. Use your head for something other than to break your next fall."
"There's this adage that we have 2 ears and one mouth so we should listen twice as much as we talk. Unfortunately with the Internet people have taken this old adage and turned it around. They have two eyes and 10 fingers so they think they need to post 5 times as much as they read. And since they have 10 fingers and one brain, they only have to think 10% of the time! "