I'll piggy back on Pettifogger's post. This little "73 project could wind up costing you a bundle. Most "I just got an incredible buy" stories don't end well. Not well at all.
I can't see inside your gun, but assuming the worst, you'll need links, pivot pin, breach block, firing pin, barrel, magazine/spring/cap, cap screw, lever side springs and screws and a Main Spring/spring tension screw.
And maybe some other parts depending on why it won't work after you put it together.
There is a fella in Trenton, Tennessee name of Buckingham who will have the parts you need. Please understand, these parts are not being manufactured by anyone, anywhere. Your gonna pay thru the nose for em.
You might want to browse the Web for "shooter" grade originals. They will most commonly be found in 44-40, 38-40 and 32-20. There are no "new manufacture" '73s being made. You will have to settle for an Italian made replica or a Japanese made replica.
Good Luck
Coffinmaker