yes packing is pretty much your only solution if you wish to stick to your original suspension. i have heard from many that corolla spirngs fit but i'll not suggest it unless you get the ratings checkd and see if they are compatible to your cars setup.
ideally you shoudl look for springs with the same diameter and number of coils, just ensure that the coils are slightly thicker (maybe 1mm more) and your problem should be solved. I can't comment further on the roll asprings without comparing them directly with the vitz springs face to face. either way by fitting CNG to yoru car it has compressed your suspension and somehow you'll need a solution.
what "packing" do you have in your car at the moment?
perhaps LOGAS has a solution for you.
congras on the purchase. any idea what that handle on the boot lid is for? i've always wondered as it makes the vitz look like a suitcase for no apparent reason/function.