Did a little research of my own this afternoon around the legalities; according to my interpretation of the RTA's documents on the subject (linked below) changing from the SOHC to the DOHC would fall under the category of "Owner Certified"/minor modifications, as the entirety of the running gear and exhaust system would be retained from whichever car the engine and tranny come from anyway due to the compatibility of the parts... So based on this, minor modifications do not require engineer certification according to this document. I guess this is good news!
http://www.rta.nsw.gov.au/registrati...s_nov_2007.pdf