swap an old engine in a newer charger?

No emissions testing?! I had no idea. With that problem aside, it's just a matter of will to do this.

I hate to suggest this but as much as we love our classic smallblock V8s, all of the tech that we loathe has been an evolution of the tech that made these engines to begin with.

Seldom do they make engines worse (late 70s not withstanding). Expect to see vastly worse fuel economy and probably not much better performance unless you drop in a hot-rodded motor...which also means giving up street manners.

I'm going to be the wet blanket here and suggest that you consider a Pentastar V6 which are all aluminum, ~300hp right out of the box and will possibly bolt right up to your motor mounts and transmission. ECU issues, I have no idea.