Many years back I had a similar situation, Removed the barrel and breach plug, Checked to see what the next size larger touchhole liner OD diameter size is available and made sure the barrel will safely accept the larger liner, drilled the barrel liner hole larger and tapped it to fit to accept the larger size touchhole liner size. Used a round piece of stainless shaft to make a new ECENTRIC touch hole liner, Chucked it in my lathe and cut threads on it to correspond with the newly tapped thread size in the barrel, chucked the stainless in a 4 jaw lathe slightly eccentric then drilled the .062" touch hole thru the stainless liner, Drilled the touchhole liner in line with the eccentric .062" touch hole an appropriate diameter to accept the 4F priming black powder, Install the liner in the barrel with eccentric .062" liner hole forward , Ground the inside of the stainless liner to insure a patched jag will not hang up on the stainless liner, cut and file the liner flush with the outside of the barrel flat, Install breach plug and barrel ect. Grind pan like Justin recommended in a previous remark ! Liner should be designed so there are several thousands of an inch that the touchhole that can be countersunk slightly on the outside to improve ignition. This will improve the touchhole relationship to be closer to the center if the pan was enlarged like Justin recommends to rear of the pan. Lots of common sense and layout design must be used to insure a safe design. GOOD LUCK!!