Zenith 28 carbs were widely used on GMC 228/248/270 engines so it may not be too hard to find a used one but there were quite a few variations within the 28/228 series for mounting, relation of choke and throtle butterflies, venturi size etc. so check that the one you are looking at fits in all ways.
Also, new 228 carbs are available. Sources include
http://www.autotestsys.com/Zenith.htm and
http://www.carbsonly.com/carbs/indfarm.htm and possibly others too. Pricing seems to be about $600.
