Yes, heat exchanger (HX) zincs need to be replaced and it would seem frequently as there can be a lot of dissimilar metals in a HX. Catalinadirect.com carries all three sizes used on various Universal engines and they cost about $10 each.
Read this page: Catalina Direct: The Unknown Zinc
and buy all 3 sizes to start with and once you know what size yours takes buy a few of them. Give the ones that are the wrong size away.
HX = extra life on your engine + extra maintenance.