You need to change not only the lock (or convert) but your hooked breach plug is a snail-bolster style that would also need to be changed over, and your existing tang has the rear end of the snail, so it need to be changed too. Unbreaching Lyman GPR barrels is not easy.
If you change barrels you could perhaps have a drop in conversion.
Buy the complete GPR barrel and the complete flint breach plug (assembled) and also the lock, all from Lyman. But I bet it's going to cost a lot.
As Kaywoodie said, it would be better to simply buy a GP rifle in flint. The cost would be close to the cost of the conversion and if you just buy the flinter you then have 2 guns.
