mx5cartalk.com

stilletto_rebel LPG'd his NA6 in the UK - might be worth pming him - he's also got a blog which provides some details on the system including the tank he used:
http://lpg-mx5.livejournal.com/

changing an MX5 to lpg legally in Vict is not quite so simple as some imagine.Talk to an lpg installer and the reality may be very different to the aspiration. Try it also with your insurer hmm.. UK and other places do not mean anything here. Why anyone would even do it is a mystery- its a sports car!! Great idea on lump 6's and 8's weighing +1600kg with authorised kits.

How much does the tank weigh?

It's got interfere with the weight balance of the car and can you get a big enough tank in there to give you any reasonable sort of distance between fills?

There must be other, less restricting ways to get a s/c certified.

zoomzoomzoom wrote:LPG will enable me to pass emission regulations as part of the certification process ...

Is this the main reason for installing LPG?
If so, then it would be counter-intuitive as Kraftwerks' kit is designed with California's EPA/"smog" legislation in mind, which is one of, if not THE, strictest in the world.
If it passes there, it'll pass anywhere.

sounds like an excellent way to burn money. Judging from comments by other forumites modding even minor is fraught with hassles- restorers building replicas cars etc are not just faced with the mechanical issues but with very expensive engineering prove outs. Modded cars also bear little resemblance in cost to their resale value - evidenced by guys trying to sell modded cars and then selling off the add on bits. Insofar as lpg systems are concerned some simply do not work properly because the cost of proving and certifying is prohibitive.