From what I have heard from those using Hatfields they shot OK but the flintlocks were kind of iffy. I do not know if it is because those that owned them were not "up" on flintlocks, or tended to use the cut flints, or becasue the locks were not very good. Track of the Wolf sells a mainspring, sear spring and complete lock for Hatfields. The complete lock would be the way to go, of course for conversion. If the touchhole does not line up from a normal insert I have made them out of stainless steel bolts and drilled them to be at the "sunset positon". I do not know what the threads are on the Hatfield drum, but are likely metric, whcih may require either matching or redrilling. As to being worthwhile, that is personal. Could you get a good kit with a straight barrel at about the same cost as the gun + lock where the barrel would be a known entity and the lock a good one? Do you want to mess with a kit? etc.
DP