I certainly agree with the others that zinc is not ideal, though for non-critical hardware, I don't see a problem with it. That is, parts that won't result in an injury should they fail. Definitely don't use it for capsquares, axles, adjustment screws, or other parts that are load bearing or safety devices. Of course, if you're going to do some of it right, why not all? You'd end up with a mis-matched carriage if you used steel or brass for some hardware and zinc for the rest. It'd be a shame to machine a nice barrel and make a custom carriage that looked strange or wouldn't allow you to live fire.
Try to fabricate whatever you need. After trying, if you fail to produce a usable part, try again. If you still can't make one, contact a sponsor. You'd be surprised at what you can come up with if you try it, and as reasonable as our sponsors are, you'll almost always save money if you can do it yourself, and it will be a point of pride that you managed to make something you didn't think you could.