A deer rifle - any chance of hunting moose or elk - or maybe Big bear?
If building merely a deer rifle, I'd go with a Marshal or Lancaster/Virginia style .50 cal. rifle in a longrifle.
The Jaeger and English-styled flint rifles were the height of design for best flintlock hunting rifles - shorter in barrel and better geometry for fast and accurate shooting. This grew into the cap-lock Sporting Rifle of the 1850's of English Design in caplock rifles - none better in design or function.
One must decide. Of all the designs of flinters, I'd go with a E. Marshal in a longer gun or perhaps better/maybe, a Jaeger with 31" bl. as being the ultimate in a flint .54 cal. hunting rifle. That size will handle future hunts for larger game than deer, yet if properly loaded, will give great power and good range for deer as well.
I think the Marshal could be built in .50 or .54, while the Jaeger should be .54 - just a preference.
Do not even think Energy nor any other modern #'s game have anything to do with a round ball's killing prowess.
I have witnessed a .535" round ball kill a big bull moose at a lasered range of 170yards. The ball was started with a fairly light charge of only 100gr. 2F- perhaps giving 1,500fps, maybe 1,600fps velocity from it's 28" bl. The ball hit perfectly, centering a rib, then lung, heart, the other lung, passed between the ribs and stopped against the hide on the off side of the moose - total penetration side to side of perhaps 30" of moose,leaving a 1/2" hole through everything. The moose ran hard for 40 yards dropping dead mid-stride, totally bleed out. What better performance could you ask for from a round ball?
You are right, the guide had no business letting the hunter take that shot - he did (Taylor nor I would have permitted it) and they got lucky with a good hit. I only print this to show the killing power of the load. The ball possessed, as near as we can figure, about 400fpe at 170yards. Modern writers state that is 600FPE less than needed to kill a deer and 1,100fpe less than needed to kill a moose. There is something very wrong with FPE numbers and subsequent statements of killing power.