Legally, it doesn't matter what caliber your lower is marked with, SBR or not, you can put any upper you want on it.
I have a pre-ban Colt Sporter in 7.62x39. I have run limited amounts of steel case thru it with no issues. Like you said, I wouldn't worry about it 'till some thing breaks.
The latest C Products 30 round mags are the ones to get. I think the ones out now are 3rd or 4th generation, & they are finally reliable.
C Products 7.62 x 39 Magazine