My opinion - take it or leave it - depending on how much cash you want to shell out is to get a powder measure and a bottle of Pyrodex RS and start working up a load with the bullet in the 250 - 300 grain range of your choice starting at 80 gr up to 110 gr.
Or you can get a breech plug designed for Blackhorn 209 and a bottle of Blackhorn 209 and do the same thing. I haven't had a .50 cal muzzleloader yet that didn't shoot a .429" - 300 gr Hornady XTP with a green Harvester crush rib sabot with good to exceptional accuracy.