I shoot both right and lefty percussion rifles. Not a lot of difference to me in the process although shooting a righty pulling the hammer back with my thumb seems more natural and easier. It is like closing a fist as the thumb is extended further across the rifle. Capping is not to difficult although on some the hammer gets in the line of sight making it difficult to see and cap. Then turning the gun to help line of sight you rotate away from your body and the hammer gets in the way more. Benching a righty for a lefty is easy peasy capping.
I own lefty flinters and much prefer those. I have shot righty flinters but priming the pan that close to my belly is not easy or comfortable to see. As well as then holding it away from my body is shaky. Priming the pan on a lefty with my left hand is much easier as the cock does not get in the way nor the barrel or line of sight. Just easier to see and get it done.
If you can find someone to put one together for you you'll be much happier with a properly stocked left-handed gun.
Key word in the sentence above is "properly".
Make sure you get the right length of pull for your body.
I find it hard to understand that thousands of muzzleloaders are being sold annually and they all have the same length of pull.
No one can tell me they all fit!
If you could actually drive to and meet the individual that's making your gun.
I'd go ahead and do it, that way you can get measurements.
(Provided the maker knows how to fit a stock)
I would buy a Kibler if they offered a left handed rifle. But as it is I'm not going to spend that much money on a gun that has the cheek piece and patch box on the wrong side, as well as cast off that is trying to make it harder to line up the sights. I'm not as concerned about which side the lock is on, but those other things are a deal killer for me.