Thanks for all the great advice.
I'm still waiting for the replacement plate ($208 + $18 shipping) and did manage to find a couple foundries (http://wiki.vintagemachinery.org/FoundrySources.ashx
) willing to recast a duplicate ($50-$80) for a spare. Most foundries no longer fool with small custom cast iron, or require a pattern which costs a couple thousand dollars to make.
I'm not handy at all, so trying to make a template for recasting out of the cracked/warped plate was not in the picture. Should've done that when the hairline crack first appeared.
Meanwhile, I've stuffed enough door gasket material into the gap between the plate and the firebox so that stove seems to be working properly. Will replace that stuffing with some scrap rock wool when I can get it later this week. Thanks again.