@mokeman, well done on your score. Sounds great and certainly something far from common. From the “topless” point of view I would consider the EJ-DE an excellent highly efficient engine that will be relatively maintenance free and for roofless cruising that will have a more tempered spirit than the original engine since it will pull well in all gears and not need to be hammered to keep moving with ordinary traffic.
Not much detail but an old picture from the old forum with an ej-de topless http://imgur.com/a/ZSB17 unfortunately not much side profile.
So your questions are based on having an unregistered car that you want to get rego for? In Qld MainRoads would prob focus a lot more on the lack of roof than the engine change, but still one can’t ignore the issue of the engine. I will assume the roofless mod is factory and has compliance. Have you checked with MainRoads for your state? You prob have and may have had “the run around”. I would almost hazard to say that if you put M100 struts in with L200 springs, M100 brakes and hubs and the L700 control arms and all was black tidy and looked factory no one would know unless you told them. But I cannot tell you to do that as such might be legal. If you did go that way, whether telling mainroads or not, it means: a 100pcd stud pattern (rear Sirion hubs go on and are spot on dimensionaly and the wheel cylinders slip in also with the only diff being a diff bore size) for a bigger selection of wheels; much better brakes (no need to change the master cylinder as the std bore size leaves plenty of fluid in reserve); a safer wheel bearing set up; and the whole thing will feel more balanced in that the more modern engine is complemented by what will feel like a more stable front end with matching brakes.