I am wondering if you have the right size agate. Normally they sit closer to the frizzen then that. If you have and Thomas Fuller black English flints, try one of them. When I put the new flint IN LEATHER and set it, the distance between the nose of the flint and the frizzen is about 1/16 of an inch. Its really tight. And the first few shots will reset that distance as the flint knapps.

I have never owned a Pedersolli but have heard a lot of good things about them. As for the threads in the touch hole, I would suspect that a Lyman thread might work. But could not guarantee it. Perhaps with a little research on your part through Pedersolli they could tell you the threads on their touch hole. Then you would be certain.