The old rusty metal strings are the key. Are you sure you're measuring the scale correctly? Sounds like an old short scale Tenor Guitar. That scale should be closer to a modern Baritone - around 20 1/4". Regal made a lot of Tenor Gutars with the short scale (as well as the regular 23").
At any rate, metal strings will give it it's best voice - that is if you think it's in good enough structural shape to handle what once was normal tension.
A classical set will put less strain on it - sometimes you have to settle for that with an antique. We build a Tenor Guitar made for classical strings, so we have a set for linear G on the 23" scale. They may be a bit loose, but if that's a correct scale measure, that's about as close as you'll get. No guitar strings will hit that sort of a scale on the money in G.
If you're not tied to an exact tuning, moving them up 1/2 step or a full step to A tuning - that should be perfect and that key sounds really nice if the body is relatively small, like a Regal.
If you want to try ours, it's the G23 (g tuning @ 23" scale - clever, huh!) Linear String Set w/ Round Wound basses.