You can get a pretty good idea as to the twist rate by running your cleaning jag down the barrel with a patch on it...That being said, its "traditional" styling so I would bet the twist is either 1-48 or between 1-48 and 1-66...Not many imports run 1-72 twist for a .54...Some have a funky twist like a 1-56...
If I'm reading that page right it probably has a 1-63, like the one in the picture, most of these manufactures have a set rate of twist for a given caliber...As long as its 1-48 or slower, you can get it to shoot well enough to hunt deer with...
At any rate, I'd get some .530 balls and some .015 and .020 patches, along with Goex FF or FFF and go to the range, I'd bet somewhere between 70-100 grains will give you good accuracy and have enough power for whitetails....