Why do you want to replace the tube? Whatever it is that's pictured in the center photo, it doesn't look compatible.
I'd go out and shoot it first and see how it works as is.
The biggest issue with assembling an AR isn't so much getting the parts together right - it's getting the right parts together.
You'll remember the quality of a gun long after you forget how much you paid for it.
US Army 1966-69, VFW Life Member, Retired Geek