which thumper?

Of the bikes you mentioned, I think the WR will be the toughest to get green stickered. They make a green sticker eligible KTM 520 (make sure it's a "G" version). I think the XR also gets a green sticker. The dirt-only version of the DRZ gets the red sticker, but you can get a street legal version if you wish (street legal bikes avoid the green sticker issue altogether).

Also note that the KTM and DRZ can be had with electric starting (I don't know about the XR). The WR is kick only and, depending on who you talk to, sometimes you will be kicking all day to no avail. I also think the WR feels a bit top heavy.

I don't know anything about making a dirt bike street legal in CA, but I suspect it's pretty tough. It may be easier to do it in another state and then bring it to CA. I'd be interested to hear from anyone who has done the conversion.

I am pretty sure the WR 400/426 gets the ugly red sticker. I know for a fact that the WR 250 gets the red one b/c my riding buddy has one.

Have you considered a KLX? I hear they are pretty good, especially with some modifications. I also think they get the green sticker.

go with the drz 400e. This bike is great! Just bought one, and cant say enough great things about it. Got mine brand new for 4600 out the door. Its real fast, handles great, and is comfortable. Way easier to ride than my KTM 250exc. And you cant beat the price for one.

The 520 is awesome. The DRZ and XR650 are right out in my opinion since they are not race bikes like the others. Too little power for too much weight. I can only imagine whoops on those tankers.

I have ridden a YZ426 and it doesn't have the same satisfying grunt as the 520.

I sure don't regret my decision to get a 520 SX. I might've gone for the CRF450 if it was a bit larger and had been around at least a year.