I am leaning toward the cap lock idea it would indeed be easier to fit than the flint.OK. Given the circumstances (like the lock shape and touch hole location) i still would go for either building a lock to fit or modifying the flint plate to a percussion. You may have to add some metal to the plate to surround the drum properly but may be less than the effort of building a new lock. Would add the drum first then modify lock to fit. Might even give it more of a Custom look in producing what would have happened back when flints went to percussion. Would love to see results.
I will show photos of the gun as I finish it.