It's been my experience that aftermarket Mag-na-porting hurts the value, unless you find a buyer who would have had it ported anyway. Good luck with that!
I doubt anyone would ever know the barrel was replaced if you can find the original length and correct vintage. Having said that, I doubt you'll recover the costs. I had S&W swap out a barrel for me about four years ago. They charged $75 to do the work and shipping to them cost me about the same. I've got to believe you'll pay $100-$200 for the barrel (if you can find one), so it would have to add over $250 -$350 to the value to make it worth the trouble.
I suggest you enjoy it for what it is or sell it for what you can get.