I couldn't find a suitable replacement bolt at local hardware stores. Most M8 bolts in stock were 1.25 mm threaded instead of 1.0 mm fine threaded, hex head instead of allen key, or not the high strength 12.9 grade steel. I did find something that looked suitable on-line: https://www.fastenal.com/web/product...ex?sku=0142067. None of my local Fastenal stores had the part in stock however, a triple whammy from the fine thread/allen key head/12.9 grade requirement.

Fortunately I was also able to find a local bike store that had the original NOS Campagnolo seat rail bolt in their older parts collection. So for $10 I am back up an riding. I'm also ordering the Fastenal version on-line, and will carry it as a spare. Probably not necessary, and I would be delighted if I never have to use it!