The important thing to remember when registering a non-CA motor vehicle in California is 7500 miles.
If the vehicle has more than 7500 miles on it, then you're golden. At that point there are no restrictions on registering it other than those that apply to any bike you buy used in state.
If the vehicle has less than 7500 miles on it, though, then state law requires you pay sales tax on it as if you had purchased the vehicle in state. Now, before everybody gets angry about them taxing you when you buy something out of state, keep this in mind: tax on big item purchases like motor vehicles is a big part of not just the state budget but also local government's budgets. The idea behind this law is to make sure that when Californians get taxed, the revinue goes to California. This law protects the reviue that pays for things like schools, roads, and off-road legal wilderness.