There are no smog tests for Diesel motors in California.  Hence if you have
a Diesel you can "get away" with doing whatever they heck you want with
it...  for example taking out a stock diesel motor and putting in a turbine
motor that runs on jet fuel if you want...

Also vehciles older than 1975 are exempt so again you can rip out whatever
the factory motor may have been and put in whatever the heck you want.

Anyone interested in the requirements for normal (non-engine change)
vehciles can read:
http://www.dmv.ca.gov/vr/smogfaq.htm

What I've never gotten a straight answer to is: Can you EVER replace a gas
motor with a diesel motor and if you do what happens?  I _think_ you can...
and if you do you then magically become exempt as if your were diesel and
then could do whatever they heck you want including taking the diesel out
and putting in whatever power plant you wanted....  I think...

If you read this:
http://www.dmv.ca.gov/vr/osreg.htm#BM2558
and
http://www.dmv.ca.gov/vr/osreg.htm#BM2558
you'll also pick up on another little tid-bit about importing non-california
vehicles.  The group currently upset about it is all the Bio-Diesel folks
who'd like to by brand new diesel but that are not offered for sale in
California... so they are buying them in neighboring states, putting milage
on them... waiting the time period and THEN bringing them in.  The market
for 1 year old diesel's in Nevada, Arizona and Oregon is flooded with
California buyers...  well "flooded" might be a bit extreme, but still it's
affected.