If the seller doesn't have current registration paperwork (not just a sticker on the plate) I ALWAYS check first. There is a calculator on the DMV website. http://dmv.ca.gov/FeeCalculatorWeb/index.jsp It doesn't always work, if there is a transaction in process you don't get any results, but always worth checking. You can actually call the DMV though if the web calculator doesn't work. Key is that the wavier of fees under the historical vehicle code only works on stuff that is 25-years old or older.
My friend complains about all his cars because he does not use them but never goes non op. I have three cars on non op and never an issue. I told him to not be so lazy, non op them and be done and stop complaining. Its not California, its the owners