I'd look on the NAA website for a parts diagram and manual. That may help. Don't FORCE anything. Usually if a gun goes together but is 'stuck' there is some wire spring not on the right side of a part, or some part that wasn't swung into position when assembled. I'd look carefully at the hand, as the likely culprit.
I always take stuff apart too - it helps know the design's 'weakness' (and demonstrates mine...).
Please give us a little better description of your problem, saying you replaced the hammer and now the gun won't work is like calling your mechanic on the phone and telling him your car isn't running good, what's wrong with it?
Can't get the hammer to fall ,can't get the main screw lined up.Can't get the cylinder hand spring to seat. Do you put the main spring in last to keep tension off? If so want tricks to get the main spring back?Very hard to work with the very tiny parts.Not many parts to deal with.I have the blow up diagram.
If I recall correctly, when I had mine apart four or five years ago I lost the small pin for the cylinder latch and that fouled things up real good.
The pin was under (and held in place by) the left grip and is about the diameter of a large size paper clip. That's what I made my new pin out of.
Hope this helps, that's about the extent of my knowledge of these guns.
Thanks to my good neighbor it is back together & working with a new mainspring.Seems as if original owner had messed with it.Anyway it works.He used some needle noise to put the mainspring in last.Best advise to all ,send it back to factory for workif you need to disassemble.