First, if you decide to build from scratch, get and study a copy of Recreating the American Long Rifle
by Buchele and Shumway. It's available on amazon.com or eBay.
Second, contact Ray Franks at Sitting Fox Custom Muzzleloaders. He has a couple of left hand long rifle kits available and caliber is optional. Here's a link to his left hand early Lancaster .36: Early Style Lancaster .36 - left hand