Repair parts Copco enameled pot
If the rod is simply...
If the rod is simply threaded on each end, it could be the sort of thing you could fix yourself if you are handy, since I would be very surprised if there is a source for parts for vintage Copco pieces. A rod in the same diameter, and a thread cutting die from a hardware store might be all that is needed. However, if you are in the U.S. the project might be complicated by a need for metric sized tools and materials.
tchp's advise is correct....
you can make a new rod yourself and the youtube link explains clearly how to do it. My only additional advise is that you do not need any metric tools if you are in North America. Just drill the hole in the cast iron part and in the Zamak end piece to the next size drill in fractions of inches. Of course you will have to cut the thread in the newly drilled hole, but to find the proper rod is a lot easier...good luck, It's worth it!
Cutting and tapping threads?
Why go to the bother and expense when there are easy sources for metric threaded rod? The hard part is determining the correct diameter/thread pitch.
An alternative might be to take it to a small engine repair specialist. They often have the tools and expertise necessary to make that kind of repair.
Yes, if the rod is simply...
Yes, if the rod is simply threaded on both ends, getting a piece of fully threaded rod in the proper size would be the most straight-forward and economical solution, as tktoo stated.
Another good source
for small quantities of small parts and fasteners is Amazon Supply, which used to be Small Parts, Inc. They have a very broad selection, and two-day USA shipping is free if you have an Amazon Prime membership.