The bigport has a lighter bottom end, You just missed out on a 4ag scorcher dizzy/ignition on ebay, I think it sold for 300 bucks, I'm using a Hyperpak ignition and it works great, I also recommended it to a mate of mine and he has it in his daily driver, A good light 4ag flywheel is a 3.6kg, You can get them off ebay if you don't wanna get yours machined.