It is odd that this subject should come up just now. Many years back we all used cap guns until we (my brother and I) got tired of our new found black powder friends making fun of us. You know, real guns don't have nipples, etc.
So we changed to all flint.
BUT, now that I am old and have many problems shooting I thought I would change my 36 cal squirrel rifle from fling to percussion. I know, it is supposed to be easier the other way around, percussion to flint. I just completed this last week.
My rifle had a large Siler flintlock on it so I found a used Siler percussion, and went to work.
I did not have to do anything at the overall lock plate inlet. But I had the flint lock inlet a little too high so I had to carefully file the recess for the drum so that it is a tight fit and any force from the hammer is actually transmitted thru the drum and unto the lock plate.
This all worked out ok.
Another problem I had was that when I originally built this rifle some years back I tried an old trick my friend Phil Cravener used to do to make small barreled guns wider. He would add a pc of 1/8" sheet brass to the bolster, thus making the rifle 1/4" wider and appearing better.
So I had done it on this flintlock and the mortice was for that. So I had to add the same thickness to the percussion lock. It took more time but it all worked out, as the photos show.
But you will not find a lock with a plate that matches your inlet mortice. Perhaps Mr. Kibler will sell you a lock plate with internals, but no frizzen or spring. Maybe make or find a flint hammer? I saw one once that used the original flint hammer and simply had a pc of steel in the jaws and it struck the cap?
Hope this helps ya in some way. It is not a fast or little project.