The last post in this thread indicates that there are no parts available:--->>> Brolin Arms .50 Muzzleloader

However new & used ones do pop up for sale.
A quick Google search for "Borlin Arms Legacy Model 50 Cal" turned up a closed ad for a Legacy that uses 209 primers.--->>> https://www.gunlistings.org/ohio-gun...y_model_50_cal.

Maybe you can locate some parts guns for sale if you're persistent enough.
If you showed us a photo of the breech plug, perhaps some other company's breech plug wrench will fit or can be altered to fit.
Does the breech plug have 2 slots in it?
IIRC Traditions plunger style breech plugs have 2 slots.
Check eBay or some other outfitter for photos of new or used ones.