Ok, here's a summary for the next new owner who asks what to look for in a spare key.
The attached photo shows my original Yanmar key. I think this type fits nearly all Yanmars that use a double-cut key. Also in the original Yanmar 'ignition' switch there is a copy of that key, on an ilco blank, that I bought on ebay. I've learned from this thread that a JD 850 key is the same thing and is less expensive than an ebay purchase.
The photo also shows a replacement switch from Hoye installed on the tractor, and its key. That switch has a shutter for the keyhole; the original switch didn't have a shutter. Its key labelled 'FII' is not the same cut as the original Yanmar key.