I looked up Safari Arms on line, and judging by the patchbox and overall appearance of the rifle it is made by Investarm in Italy.
I owned one in .54 caliber, and this is supported by one that I found for sale in .54 caliber.
In the links below, the Safari Arms and Investarm rifles are the same, and are still made and sold.
https://joesalter.ca/products/50-cal-hawken-sold-by-safari-arms
http://www.investarm.com/eng-avancarica.html
https://www.dixiegunworks.com/product_info.php?products_id=3461&osCsid=ldlds1jl06llhkb0e0t28mhd81
Also, the Lyman Trade Rifle is made by Investarm, and while it has a fixed rear sight and a single trigger, the barrel is quite likely the same otherwise.
http://www.midwayusa.com/product/1064883684/lyman-trade-muzzleloading-rifle-percussion-1-in-48-twist-28-barrel-blue
So, if the .54 barrel is the same width as the .50 barrel, perhaps a Lyman Trade Rifle barrel would just drop in and the key would line up with the barrel retainer.
This is very likely since my Lyman Great Plains .54 barrel is the same size as my TC Hawken .50 barrel.
Getting one out of the USA would be the hard part I would assume.