[Arran]

It's possible that the red wire, actually a grid cap lead, is a very old replacement, I can't remember seeing a pre 1939 Philco where they used the rubber/gutta percha covered wire for grid leads, that was more of and RCA thing. As to what to use as a replacement, well it is low voltage, and low current, but you also want something mechanically sound, so 10 or 20 gauge, stranded, would do
Regards
Arran