I think making a kit in styrene is beyond most of us kit builders. You cannot simply melt plastic on the stove and get it liquid enough to pour into a mold. A styrene kit like we build is injected plastic. This means that plastic pellets are liquified then under immense hydralic pressure is injected into a mold that is loaded into this big expensive injection molding machine.
Now however you could make a vacuform plastic kit in your basement but again you need the right type of equiptment.
Also the guys who do figure kits in what is called the Garage Kit industry make their kits out of resin. You could make your pattern then slice it up into individual components for molding. You then use Room Tempurature Vulcanizing rubber to pour over the pattern to make a mold. More than likely it will have to be at least a two part mold and you must be very careful of any undercuts in the piece otherwise it will be stuck in your mold! You then use resin to pour into the mold to make your pieces. This is a skill I wish I had more knowledge on because RTV rubber and resin aint cheap and your failed pieces cost money.
I hope this helps,
Cheers and good uck if you pursue this, Max Bryant