Hi Doug, according to my sources red rye malt powder is not a diastatic malt powder, so barley malt syrup would be an adequate substitute, but certainly wouldn't contribute the same flavor or color. I suspect it's also a bit sweeter than the rye malt powder, so I would suggest substituting 2 tablespoon barley malt syrup and perhaps adding an additional tablespoon or two of rye flour to adjust for the added liquid the syrup will bring. 

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