The firing pin you might be able to fab out of an appropriately sized bolt by using a drill incorrectly (as a lathe) or by simply tapping your own threads through the bolt and running your own pin replacement in the form of whatever bolt is convenient. I did something similar with a spump build a while back by running a 1/4-20 hex bolt through the marker's bolt then grinding the end down so it fit the hole on the hammer. Held up OK.
As for the feed neck, look around for plumbing supplies and get creative. Hose clamps solve like 90% of all problems.
I bet if you ask around at your local field you can get someone to dig a spyder threaded barrel out of their bag for you for a fiver.