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...Another free option is GIMP - it has things Photoshop lacks and vice-versa and has very good support from the user groups, and, depending on where in the world you are the answers can arrive in minutes....
In my case, I simply took a dozen of pictures for a sheet of drawings, totally and partially, as I was going to leave them as CAD data finally.
As an option, contact your local university, as many have vintage map archives and the ability to scan vintage maps such as the old USGS survey maps, which were fairly large.
Contact Ester at estera@aircorpsaviation.com.
She is doing this sort of work all the time. I have dealt with her on other matters and found her to be extremely helpful.
Her video at Ken Jungeberg Collection | AirCorps Library may give you some clues.