I made a slave pin out of the tip of a brass punch the correct size. A correct dia. nail would also work. Cut it so it is a hair shorter than the width of the hammer. I took a file and rounded off the edges of the slave pin.
After you have the pin made, assemble the spring and hammer together. Install into the frame and line up the hole. Drive the frame pin into, pushing out the slave pin. You will have to hold the hammer in just the right point to get everything to line up.
The hammer spring goes one way. If you install the hammer and it feels easier to pull back then before, you have the spring upside down.