I have a 1963 model 12 in perfect condition.
Modified choke barrel.

Can I use any of the non-lead shells and take it duck hunting, or would I harm the barrel shooting any shot which is non-lead.

You should be fine with all the lead alternatives with the possible exception of Tungsten Super Shot. (TSS)
However, it may be wise to have your barrel miked to ensure it is Modified and there has been no mistake. It has been known to happen.

Most choke tubes are rated for steel up to modified constriction. But most choke tubes are thicker and probably have better metallurgy than your model 12 barrel so, I would use it but take it easy and shoot smaller shot (maybe up to #2) and some of the softer pellet alloys such a bismuth to be on safe side.