Thanks guys for the info! I'm assuming the original HGM-C's are repairable if needed. I'm banking on the HGM E's will outlast me and my soon-to-be 28 year old mower.
There are a lot of the IS1500's still working commercially in my area. I will offer to sell the C's to a yard guy as back-ups. I'm sure they would work for a while before a re-build. With this style motor being obsolete and the Ferris mowers still in good operating condition, it's a matter of time someone will need one or both. I can give them info on the parts kit and let them decide. Just hate to throw them away.
One thing I learned on this thread is it must not be a lot of fun to be in the repair business with all the supply chain nonsense!