Ed,
If all else fails, you can do what I've been doing for a few years...
Find an ACE, Lowes, Home D or other major hardware store that sells US and Metric keys. Bring along the engine, and try what they have until you find one that fits right.
The local Ace guys are used to me by now; you might have to break in some newbies...
Now, if THAT fails, there are specialty fastener shops around. If none local, S&H is guaranteed to be more than the items ordered... If they can even match from the bolt specs.
BTW, the key that came in the box with mine (China-made) measured about 2.9mm across the flats. A 3/32" key is too small; that's 2.4mm in metric.
Someplace to start, anyway.