Since you're looking for original action parts, you might consider a Super Blackhawk set if you like the wide hammer and/or wide trigger They will work just as well in your flat top. If you switch to wide trigger, of course you'll need to widen the slot in the trigger guard, which is not hard because it's aluminum.
Or you can switch to an steel NM XR3-RED size grip frame which already has the wide slot and 1/8" more knuckle room behind the trigger guard for those 45 'T-Rex' loads! If interested in that, let me know for pointers on adapting it to the old model, pretty simple.
The only original parts you really need are the cyl latch and hammer. The cyl pin you have will work. The trigger just needs the transfer bar lever cut off and the bump behind the trigger filed flat for it to work.
Old Model flat top converted to 44 Spl, SBH wide trigger, and stainless new model XR3-RED size stainless grip frame:
Old Model 357, SBH hammer, SBH wide trigger, and stainless new model XR3-RED size stainless grip frame: