I bought a Saiga .308 rifle in 1998.  I never new that in 2000 it was mandatory to register all assault rifles.  Is there anyway for me to register now.  Am I breaking the law by simply possessing this item?

register it now? NO
simple possession illegal? YES

safest thing to do is move it out of state, i dont believe there is anyway to modify it to make it usable and legal.

Send it to me I will dispose of it for ya

You have a problem.  According to the AG Site ag.ca.gov/firearms/regagunfaqs.htm#15 your only legal option under State law is to turn it in.  AFAIK the only people that ended up being able to register after the date were some of the guys that went through the yes it is , no it isn't routine with some of the SK variants.

You might want to check with the Feds to see if you can sell it out of state.  In other words take it to Nevada or AZ and have a dealer there sell it or buy it.  The fly in the ointment being that you no longer legally possess it i accordance with State laws.  Whether that effects a dealer or purchaser in another state isn't mentioned.  And if that's possible, drive safely to the border and don't get stopped.

Or you can keep your nose clean forever, never do anything that might get your house searched, never tell anybody that you have it and especially never tell a buddy or girlfriend or wife about it.  ANYBODY who might get momentaily or permanently pissed enough at you to rat you out on it.  Or move.

or leave it where a bad friend might steal it and then rat him out on it.