Hey Cory, if you find a kit, and they do come up every now and then on ebay and such, I would not suggest using the Porsche seat. Depending on the year it was manufactured, the seat may now be very outdated and there are a ton of very good seats out there that will fit in the boxster if you install the deactivation bar. If your child is of the age where he can sit forward facing, I can not suggest any stronger the Recaro Young Sport. It has excellent padding + support for you kid, transitions into a booster when they get old enough, firs into the contours of a porsche seat without marring the leather, and is one of the few seats to be side impact rated and tested. Here's some pics of my son in his.
It comes in several different color combos, mine is the midnight dessert color, black with tan center. Matches the two tone color combo in my Subaru and still goes good in my black interior for the boxster. Search around on the web and you can find them new for less than 200 opposed to the 250 retail price. PM me if you want the info on the store I bought mine from.
Steve