Alas, all I needed to to was call my local Yanmar dealer (in this case; Gallery Marine, Northlake Way, Seattle). Good news: they had the standard key in stock - $15 and I was on my way with little bag containing 2 spares. Possible bad news: every 3HM35F on the planet shares the same key.
Sooo, I wonder if this same-key situation applies to all Yanmar (or other make) engines?

Happy boating to ya!
Pat