It may be worth while contacting the County archaeological unit or English Heritage, who will probably have digital copies of these maps. Otherwise there are several sources for photocopies the Museum, Cornish Studies Library & the County Record Office. This would entail copying from fiche, all of which results in distortion from the original.
I have tried using a modern digitised 25" and then scanning and redrawing the first edition as a seperate layer, as a base for adding information such as archaeological features, mine plans, & modern underground surveys. Verytime consuming. GIS would be useful, but probably out of reach unless you are doing it on a professional basis.