I knew a continuously expanding "cone" is ideal for more performance and if I could have it my way I'd like to go with a full system. There aren't too many options for first-gen ex's. Their either discontinued or used (which isn't a terrible option). I looked at your comparison and had a thought. As long as the exhaust port side of the system is the same diameter or close would any aftermarket full system work? I saw some of the ones you pictured were like that. The only difference I saw was the header length and the route they took towards the back of the bike. That maybe be a stretch but I am generally clueless when I comes to exhaust systems/ exhaust compatibility and I don't have access to a welder so I can't really piece something together. I guess the biggest question is:has the basic shape of the 250cc engine changed that much over time to where I couldn't make it work?
I also like your idea of mocking it up from the muffler towards the front of the bike...