During the brief period where cable guides were under the bottom bracket and the old type plain steel variety, IME the plastic liner guides were always used. Usually this lining had little flanges on the ends so it would not slip out, but you could try instead very long pieces.

However, it looks a little late for that, as there is already a notch ground into the guides. I think the plastic would get stuck there.

To my eyes, it looks like the guides were badly placed by the builder of the bike. They are too far back.

Depending on the value of the bike, IMO the best solution would be to grind off the guides, touch up the paint so it doesn't rust, and install a screw on plastic cable guide. If you prefer not to alter the bike, figure out some way to run old style stainless housing over the worn out guides. Maybe fill the worn notch with epoxy 'metal' putty first. You'll probably need to secure the cable guides with a clamp of some sort.