I know you can't buy this blank anymore, but is there a blank available with a similar action?

Of course, the actions that the HMG have/had are the same ones most all blanks are based on: fast, medium or slow. Some of the HMG casting models were very tip oriented and likely listed as extra fast. So that is what you would want to look for when trying to match something up. Like most blank lines, the HMG's were not all the same, some were extra fast, some fast, some medium and some slow. So you'll have to look for the action which matches the particular HMG model you are talking about.

Of course, if you want the exact feel there will be a bit more to it as the tip and butt diameter, material and wall thickness all have to taken into account. But somewhere there is something similar.

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I guess I should specifiy the HMG fly rod blank.

The older/original USA made HMG fly blanks were definitely stiffer/faster than the imported HMG blanks made in later years. I still have 1 or 2 of both types. In later years, the Royales and Iron feathers were Fenwicks faster blanks, with the Iron Feathers the faster of the 2.