CRG definitely has an extended front porch kit available.  It’s priced at $345 at Acceleration Karting for a new kit.  With that being said, I’ve seen a lot of tall drivers run without an extended porch just fine (you have the option to remove the foot rests to gain an extra few inches).

I have a complete CRG shifter kart for sale just in case you are interested… ;)

Take a look at this CKR we put together a few months ago for a 6’4″ driver. Extended porch improved his lap times. Let me know if I can help! http://www.eshifterkart.com

Be sure you need an extended pourch. My son and I are 6’2″ and 6’3″ and have run the extended pourch and liked it better without. There are thing to do to a chassis with out using the extended pourch. Seat and pedals. We run Birel chassis.