A motorcycle that doesn't meet California emissions standards (but meets US standards) can be registered in California only if it has at least 7500 miles on the odo. There are exceptions for residents moving in from out-of-state.California will be tacking on a Emissions Fee
to the price. Haven't heard the $$ yet.
Not sure what would happen if a person from California
bought a SS out of State, what they would say.
Or if they would disallow it.
If you don't have to go in for a smog test you can do what you want as long as it's not loud enough enough to draw undue attention.Any motorcycle after 2013 cannot modify muffler or make surpass certain db. noise. Next is smog test for motorcycles in California.