when you modify a car does it need to have the parts registered or have some kind of license or certificate?

negative. (at least in CA)

In CA the only thing you need to make sure of is that your parts have a CARB/EO number. And that's only an issue if a cop stops you and wants to look under the hood. As far as registering goes, they don't really care.

If you put a lot of work in to it, you might look in to getting some extra equipment coverages on your insurance though.

I know a guy that distributes HID kits and he gives out certificates that show that the kits are legal so in the case a cop would stop you, you could just show him and he'll lay off all the paperwork n junk.
N you can be like:
"haha! told you PIG!"
and he'll be like:
"sorry sir"
and you:
"now get out of here before i backhand you..."
and he'll be:
"yes sir...thank you sir..."

:monkey:

p.s. not intended for subaru cop owners.